I'm an independent practitioner of the Lotus Sutra along with studying Nichiren's Gosho - not affiliated with any school of Nichiren.
In the Lotus Sutra, Shakyamuni Buddha expounds that during the latter day of the Law the Votary of the Lotus Sutra will make his advent during the 5th five hundred period after his parinirvana, in which Buddha Shakyamuni instructs us that the only effective practice for the 'Evil' age is that of the Lotus Sutra and entrusted the Correct Law (Lotus Sutra) to the Bodhisattvas of the Earth (his original disciples) to be expounded by Bodhisattva Superior Practice... this coincides with the period in which Nichiren shonin emerged, produced the banner of propagation (Gohonzon) and chanted Namu Myoho Renge Kyo.
In the gosho 'The Real Aspect of the Gohonzon
' Nichiren states that the gohonzon is not his invention.
"Now, over two hundred years have passed since the beginning of the Latter Day of the Law. How awesome that Nichiren was the first to inscribe this great mandala as the banner of propagation of the Lotus Sutra, when even such great masters as Nagarjuna, Vasubandhu, T'ien-t'ai and Miao-lo were unable to do so! This mandala is in no way Nichiren's invention. It is the object of veneration which perfectly depicts Lord Shakyamuni in the Treasure Tower and all the other Buddhas who were present, as accurately as the print matches the woodblock.". . . Therefore this Gohonzon is the supreme mandala never before known, for it has not appeared until more than twenty-two hundred and twenty years after the Buddha's death. . . . Never seek this Gohonzon outside yourself. The Gohonzon exists only within the mortal flesh of us ordinary people who embrace the Lotus Sutra and chant Namu-myoho-renge-kyo. The body is the palace of the ninth consciousness, the unchanging reality which reigns over all life's functions. To be "endowed with the Ten Worlds" means that all the Ten Worlds without exception are contained in the one world of Buddhahood. That is why the Gohonzon is called a mandala. Mandala is a Sanskrit word meaning "perfectly endowed" or "cluster of blessings." The Gohonzon is found in faith alone. As the sutra states, "Only with faith can one enter Buddhahood."
If you study the closing sutra of the Threefold Lotus Sutra (Meditation on Bodhisattva Universal Worthy), you will see the outline for the practice purifying the six-sense-organs.
In the opening, Ananda asks the Buddha;
World-honored One! After you have passed away, O Tathāgata, how do living beings produce the bodhisattva mind, practice in accordance with the comprehensive sutras of the Great Vehicle, and, with right mindfulness, bring their thoughts into the realm of one reality? How do they avoid losing sight of the aspiration for ultimate enlightenment (bodhicitta)? Moreover, without cutting off worldly passions and without abandoning the five desires, how do they achieve purity of the sense faculties and eliminate accumulated impurities?
The Buddha addressed Ānanda:
Hear me clearly! Hear me clearly, and consider this well! In the past, on Mount Vulture Peak and at other places, the Tathāgata has already expounded the one genuine path (The One Vehicle of the Lotus Sutra) from many perspectives. And now, at this place, for the benefit of all living beings in the future who wish to follow the Supreme Way that is the Great Vehicle—and who wish to learn and follow the practice of All-embracing Goodness, I will now expound the method4 for that, which I have kept in mind. Impurities, in any number, should be eliminated whether one perceives All-embracing Goodness or not. This I will now explain to you, accordingly, in great detail.
O Ānanda! All-embracing Goodness Bodhisattva was born in the Pure Wondrous Land in the east. Aspects of that land have already been thoroughly detailed in the Dharma Flower Sutra (Lotus Sutra), and these I will now outline and explain. . . Ānanda! When monks, nuns, laymen, laywomen, heavenly beings (devas), nāgas, others of the eight classes of ever-present guardian spirits, or any living beings are internalizing the Great Vehicle sutras, practicing in accordance with the Great Vehicle, aspiring to a Great Vehicle consciousness, and would be pleased to see an embodiment of the bodhisattva All-embracing Goodness, take joy in seeing the stupa of Abundant Treasures Buddha, be happy to see Śākyamuni Buddha as well as buddhas that emanate from him, and be glad to achieve purification of the six sense faculties, they should learn this way of contemplation. Beneficial effects of this contemplation are the elimination of encumbrances and the perception of extraordinary and wondrous things. As a result of resolutely internalizing and keeping faith with it, and wholeheartedly pursuing mastery of it, a practitioner will become continuously conscious of the Great Vehicle without immersion into a specialized focus of mind.
... You now internalize and keep faith with the Great Vehicle—the treasury of an ocean of beneficial effects. You see the buddhas in the ten directions by reason of this cause! The stupa of Abundant Treasures Buddha appears to you as proof! Accordingly, you must express your errors and faults yourself, and amend yourself of impurities!
... The Buddha said to Ānanda: When followers of Buddha undertake to amend themselves of unwholesome
and harmful karmic acts after the Buddha has passed away, they must resolutely internalize and recite the Great Vehicle sutras. This comprehensive teaching is the buddha eye of the buddhas, it is the means by which buddhas perfect the five kinds of vision, and, from it, the Buddha’s three manifestations arise. This is great evidence of the Dharma, and it attests to the realm of nirvana: it is within such a realm that the Buddha’s pure threefold manifestation is able to come forth.
Then, expounding further, the World-honored One spoke in verse:
When the sense faculty of sight is corrupted
By karmic encumbrances that make it impure,
You must resolutely internalize the Great Vehicle
And ponder its ultimate principle!
This is called doing self-amendment for the eye
To bring unwholesome karmic influences to an end.
The sense faculty of hearing gives ear to disruptive sounds
And spoils your sense of accord.
Because such confusion occurs,
You become just like a foolish monkey.
You must resolutely internalize the Great Vehicle
And contemplate the emptiness and formlessness of all things!
You will lastingly bring an end to unwholesomeness
And hear in all ten directions with a celestial ear!
The sense faculty of smell has attachments to scents
And, so affected, drives you to make contacts.
The nose is thus crazed and seduced,
And, so affected, begets impure perceptions.
When you internalize the Great Vehicle sutras
And contemplate the true reality of all things,
You will lastingly part from harmful karmic actions
And, in lives to come, not produce them again!
The sense faculty of speech promotes the five wrong views—
An unwholesome karmic cause resulting from the wanton use
When you aspire to effect self-control,
Diligently foster a heart of compassion!
Reflect on how the tranquil true reality of all phenomena
Has no aspects for you to discern!
The sense faculty of the mind, just as a monkey,
Takes not even one moment of rest.
When you aspire to govern it,
You must diligently internalize the Great Vehicle!
Focus on the buddhas—on their fully awakened embodiments,
With the capabilities and dauntlessness they have achieved!
The material body, the agency of actions,
Behaves like dust blown about by the wind:
Six thieves have their way within it—
Without limit and free from control.
When you aspire to end this inferior condition,
To lastingly part from overwhelming desires,
To abide always in the city of nirvana,
And to be serene and have a calm mind,
You must internalize the Great Vehicle sutras
And turn your mind to the mother of bodhisattvas!
Countless surpassing skillful means are gained
By reflecting on the true reality of all things.
These six ways
Are thus named the governing of the six sense faculties.
The ocean of all karmic encumbrances
Is produced by deluded perceptions.
When you aspire to amend yourself of them,
Focus on the true reality of all phenomena while sitting
upright and properly!
All impurities, like frost and dew,
Can be dispelled by wisdom’s sun;
Accordingly, with utmost dedication,
Do self-amendment for the six sense faculties!
So, MyoHo RenGe Kyo is Theee Great Vehicle Sutra. Internalizing the Great Vehicle Sutra is the Contemplation/reflection on the very essence of your life and actions, aka Gohonzon via the purification of the six-sense organs that obliterates fundamental darkness, aka delusion. The Gosho 'On Attaining Buddhahood in This Lifetime' reads.
...if the minds of living beings are impure, their land is also impure, but if their minds are pure, so is their land. There are not two lands, pure or impure in themselves. The difference lies solely in the good or evil of our minds.
It is the same with a Buddha and an ordinary being. When deluded, one is called an ordinary being, but when enlightened, one is called a Buddha. This is similar to a tarnished mirror that will shine like a jewel when polished. A mind now clouded by the illusions of the innate darkness of life is like a tarnished mirror, but when polished, it is sure to become like a clear mirror, reflecting the essential nature of phenomena and the true aspect of reality. Arouse deep faith, and diligently polish your mirror day and night. How should you polish it? Only by chanting Namu-myoho-renge-kyo.
Thus, the Mandala depicts this very essence and is the object of veneration we focus the six-sense organs on and recite the Daimoku.
writes in his book 'The Dharma Flower Sutra (Lotus Sutra) Seen Through The Oral Transmission of Nichiren Daishonin
on the The Threefold Transmission on The Fundamental Object of Veneration
(1) Namu Myoho Renge Kyo -- which means (NAMU)
to devote our lives and found them on (MYOHO)
the Utterness of the Dharma [entirety of existence, enlightenment and un-enlightenment] (RENGE)
permeated by the white lotus flower-like mechanism of cause, concomitancy and effect (KYO)
in it's whereabouts of the Ten [psychological] realms of dharmas [which is every possible psychological wavelength].
A Transmission on the Fundamental Object of Veneration (gohonzon)
It is pointed out that these five mental images (gokei) for Myōhō Renge Kyō are what really constitute our own bodies and persons. In the teachings derived from the external events of the Buddha Shākyamuni’s life and work, these five images were either indicated as separate entities, or they were generally lumped together and included in the explanations of our objective surroundings. In this sense, these five mental images as separate entities are applicable to the wisdom of Buddha Shākyamuni’s initial enlightenment in Buddhagāya [which originated from and became the Buddha enlightenment of the original archetypal state]. Here we have, in the Second Chapter on Expedient Means of the Dharma Flower Sutra (Hokke-kyō) the phrase, “the wisdom and discernment of all the Buddhas is exceedingly deep and immeasurably profound”, and then the sutric text continues right on to the real aspect of all dharmas.
This passage in the sutra is explained by saying that the objective environment of the Buddha is like a yawning abyss, without any boundaries to it. Therefore, it is exceedingly deep, and the water of the Buddha wisdom can only be fathomed with difficulty, which, to quote the sutric text, is immeasurably profound. Again, it can be said that all dharmas are his objective surroundings and that their real aspect is represented by the wisdom of the Buddha.
At the time of the Eleventh Chapter on Seeing the Vision of the Stupa made of Precious Materials, the Buddha Abundant Treasure (Tahō Nyorai, Prabhūtaratna) represents the objective surroundings of the Buddha, and that the Buddha Shākyamuni stands for the wisdom to be able to understand them. Both of these Buddhas are seated in the same stupa, so as to indicate that our objective surroundings and the wisdom to perceive them are not separate entities.
Here, for instance, the all-embracing, wise the Bodhisattva Mañjushrī (Monjushiri) is present in the First and Introductory Chapter and, therefore, represents the whole subject matter of the sutra; the Bodhisattva Universally Worthy (Fugen, Samantabhadra) appears at the end of this sutra and represents the propagation of this canonical text. After the testimony of the Buddha Abundant Treasure (Tahō Nyorai, Prabhūtaratna), in the Chapter on Seeing the Vision of the Stupa made of Precious Materials is where the two Buddhas [Shākyamuni and Abundant Treasure] are seated in the one stupa. This expresses the significance of the Fundamental Object of Veneration (gohonzon).
Coming to the point, what do all these various deep meanings imply? – They are only the subjectivity and objective environment of the five ideograms for Myōhō Renge Kyō, whose meaning is the entirety of our existences permeated by the underlying white lotus flower-like mechanism of the interdependence of cause, concomitancy and effect in its whereabouts of the ten [psychological] realms of dharmas.
Since our physical existence, in terms of the Tathāgata Abundant Treasure (Tahō Nyorai, Prabhūtaratna), and our mentally subjective existence, in terms of the Buddha Shākyamuni, have already been discussed, the part of us that is incapable of words is represented by the Tathāgata Abundant Treasure (Tahō Nyorai, Prabhūtaratna), who does not explain the Dharma. And the part of us that expresses opinions is represented by the Buddha Shākyamuni, who does expound the Buddha teaching. The explanation for this is as easy to see as a mango in the palm of the hand; it is obviously the essence of the Dharma Flower Sutra (Hokke-kyō).
This essence is the five ideograms for Myōhō Renge Kyō that is the fundamental source of our existence. This gateway to the Dharma is esoteric and should never be divulged to anybody.
Next, there are the two wrathful emanations of enlightenment (myō’ ō, vidyārāja). The Sovereign Tainted by Sensuality  (Aizen Myō’ ō, Raga-rāja Vidyā-rāja) is the entity that represents that our troublesome worries are not separate from and can lead to enlightenment. His red colouring stands for our love of beauty and our sexual desires. That is to say that this wrathful emanation of enlightenment is the way we look upon beauty and carnal desires. The Sovereign of Immovable Wisdom  (Fudō Myō’ ō, Achala Vidyā-rāja) is the entity that represents that our cycles of living and dying are not separate from and can lead to nirvana and is one of the psychological forces that help practitioners overcome obstacles and negative energies that hinder Buddhist practice. The blackness of his colouring represents the eternally inaccessibly black karma within the realms of dharmas, which is impossible to renew and is, therefore, the Sovereign of Immutable Wisdom (Fudō Myō’ ō, Achala Vidyā-rāja).
Nevertheless, the Wrathful Sovereign Tainted by Sensuality (Aizen Myō’ ō, Raga-rāja Vidyā-rāja) bestows loving-kindness, whereas Fudō Myō’ ō represents the psychological immobility of one’s perfect absorption into the one object of meditation (samādhi).
What then do these two dharmas of our loving-kindness and mental immobility represent? They are the two dharmas of our subjective perception and our respective objective environments.
So what then are the two dharmas of our subjective perception and our respective objective environments? They are simply where we stand in terms of the Utterness of the Dharma (Myōhō, Saddharma).
By this we mean that, by enjoying and taking pleasure in the sutra, as well as appreciating it, we then become capable of discriminating with discernment a thousand myriads of different ways of speaking and skilfully putting concepts into words, so that we can explain it in a way to induce people to hold to it.
It is said that, when the Buddha entered into the absorption of the Sutra on Implications Without Bounds (Muryōgisho zammai, ananta nirdesha pratishthāna samādhi) [the meditation on the infinite meaning of existence (Nam Myōhō Renge Kyō), which is the samādhi into which the Buddha entered before expounding the Dharma Flower Sutra (Hokke-kyō)], neither his body nor his mind stirred.
When Aizen and our troublesome worries (bonnō, klesha) disturb us, we should then recite the title and theme (daimoku), in order for the Sovereign Tainted by Sensuality (Aizen Myō’ ō, Raga-rāja Vidyā-rāja), who is an emanation of enlightenment, to lead us towards a deeper understanding. When we are taken aback by the rotations of living and dying, then we should contemplate and mentally take in the implications of the Fundamental Object of Veneration (gohonzon), so that the Sovereign of Immovable Wisdom (Fudō Myō’ ō, Achala Vidyā-rāja), who is a wrathful sovereign that is an emanation of enlightenment, can teach us that nirvana implies the way existence works.
Apart from the Sovereign Tainted by Sensuality (Aizen Myō’ ō, Raga-rāja Vidyā-rāja) and the Sovereign of Immovable Wisdom (Fudō Myō’ ō, Achala Vidyā-rāja), there is no other meaning of existence – only our physical and mental bodies, their subjective perception and objective environment, along with our wisdom and the directions where our minds lead us, as well as all that takes our fancies. All this is the Utterness of the Dharma (Myōhō, Saddharma). The rest you already know.
Here are some segments from the first teaching Nichiren Daishonin wrote to one of his students titled 'On Attaining Buddhahood in This Lifetime
IF you wish to free yourself from the sufferings of birth and death you have endured since time without beginning and to attain without fail unsurpassed enlightenment in this lifetime, you must perceive the mystic truth that is originally inherent in all living beings. This truth is Myoho-renge-kyo. Chanting Myoho-renge-kyo will therefore enable you to grasp the mystic truth innate in all life.
The Lotus Sutra is the king of sutras, true and correct in both word and principle. Its words are the ultimate reality, and this reality is the Mystic Law (myōhō). It is called the Mystic Law because it reveals the principle of the mutually inclusive relationship of a single moment of life and all phenomena. That is why this sutra is the wisdom of all Buddhas.
...Nevertheless, even though you chant and believe in Myoho-renge-kyo, if you think the Law is outside yourself, you are embracing not the Mystic Law but an inferior teaching. “Inferior teaching” means those other than this [Lotus] sutra, which are all expedient and provisional. No expedient or provisional teaching leads directly to enlightenment, and without the direct path to enlightenment you cannot attain Buddhahood, even if you practice lifetime after lifetime for countless kalpas.
...If you seek enlightenment outside yourself, then your performing even ten thousand practices and ten thousand good deeds will be in vain. It is like the case of a poor man who spends night and day counting his neighbour’s wealth but gains not even half a coin. That is why the T’ien-t’ai school’s commentary states, “Unless p.4one perceives the nature of one’s life, one cannot eradicate one’s grave offenses.”
This passage implies that, unless one perceives the nature of one’s life, one’s practice will become an endless, painful austerity. Therefore, such students of Buddhism are condemned as non-Buddhist. Great Concentration and Insight states that, although they study Buddhism, their views are no different from those of non-Buddhists.
...What then does myō signify? It is simply the mysterious nature of our life from moment to moment, which the mind cannot comprehend or words express. When we look into our own mind at any moment, we perceive neither color nor form to verify that it exists. Yet we still cannot say it does not exist, for many differing thoughts continually occur. The mind cannot be considered either to exist or not to exist. Life is indeed an elusive reality that transcends both the words and concepts of existence and nonexistence. It is neither existence nor nonexistence, yet exhibits the qualities of both. It is the mystic entity of the Middle Way that is the ultimate reality.
Myō is the name given to the mystic nature of life, and hō, to its manifestations. Renge, which means lotus flower, is used to symbolize the wonder of this Law. If we understand that our life at this moment is myō, then we will also understand that our life at other moments is the Mystic Law.4 This realization is the mystic kyō, or sutra. The Lotus Sutra is the king of sutras, the direct path to enlightenment, for it explains that the entity of our life, which manifests either good or evil at each moment, is in fact the entity of the Mystic Law.
I sincerely hope this information helps provide you with some deeper insight into the True Buddha Dharma of Shakyamuni and what Nichiren Daishonin is conveying based on the teachings of the Lotus Sutra!
Namu Myoho Renge Kyo.