I like the Lam Rim seven stage method.
First meditate on the equality on all beings, since they have all harmed and helped you so many times in the past and will again in the future, their current status of friend or enemy becomes unimportant. Logistically, friends and enemies still exist of course.
“It is not the notion of friend or enemy that you need to stop but the bias that comes from attachment and hostility, which are based on the reason that some people are your friends and others your enemies.”
- Lam Rim Chen Mo eng v02 pg. 37 tib pg. 300
Then consider that for every life, you needed a mother. Therefore, all beings have been your mother, not just a few times but a infinite number of times.
Now consider the kindness of your mothers, to paraphrase Garchen Rinpoche: "Your mother carried you in her belly. She created the two things you consider the most precious things in the universe for your sake and gave them to you as a gift: your body and your life." You may want to start with the kindness of your mother in this life and extend it to your previous mother and so forth.
You might object, saying: "She did not mean to benefit me". To paraphrase HHDL: "The sun, your food, your treasured belongings do not intend to benefit you either. Yet you are grateful for them and think fondly of them."
You might object, saying: "Those kindnesses were in past lives, so they don't matter." Phabongkha Rinpoche said:
“Your mere non-recognition of a being is no reason for it not to have been your mother; nor is that being no longer your mother just because she was your mother in the past. If that were the case, the mother of the earlier part of your life would not be your mother in later part of your life; nor would yesterday’s mother still be your mother, because that, too, was in the past. This is a subtle facet of impermanence and is difficult to realize if you have not done any study. Take this shawl, for example. A year ago when it was new, it wasn’t stained, and it had no holes. But its continuation, the shawl of today, smells and is moth-eaten. This is the distinction between the two. And so, you must think over the fact that there is no difference between mothers of part and future lives. Similarly, if someone had saved your life last year you would surely remember that kindness this year!
Here is the criterion that determines whether or not you have developed this understanding after repeated training: if you see a sentient being – even an ant – you will involuntarily remember that you were once that being’s child and completely dependent on it for all your needs.”
-Liberation in the Palm of Your Hand pg. 575
Abide in this feeling of love.
Generate the desire to repay your mothers.
Reflect on the sufferings of Samsara (found in the Medium Scope of the Lam Rim) and develop compassion for your mothers who suffer.
“Here are the criteria for having developed the great compassion in your mind-stream. You have developed it if, even while eating or drinking, you think about all sentient beings, and want them all to be free of suffering just like a mother worrying about her favorite child who has been struck down by a virulent disease.”
-Liberation in the Palm of Your Hand pg. 582-583
Then think, I shall attain full, complete, and perfect Enlightenment, and by myself if necessary, place all mother sentient beings in that same state and thus permanently place them beyond all suffering.
“When you experience the conscious desire to achieve Buddhahood for the sake of sentient beings, you have developed a bodhicitta that resembles the outer layer of a piece of sugar cane. But if, in addition, you experience an involuntary desire to achieve full enlightenment for the sake of any sentient being you see, you have developed true bodhicitta in your mind-stream. You have entered the Mahayana path of accumulation and begun the three great aeons of amassing [the two collections]. You will acquire infinite qualities and gain the names ‘Child of the Victorious Ones’ and ‘Bodhisattva’. When you practice after achieving this insight, you are sure to be enlightened quickly even if you do not resort to the tantric path.’”
-Liberation in the Palm of Your Hand pg. 586-587
Of course, we should practice each stage by itself for months (or however long it takes) so that 1. we have Bodhicitta all the time and 2. it is powerful and not faked Bodhicitta.
Love is the only cause of happiness.
Equanimity is the ground. Love is the moisture. Compassion is the seed. Bodhicitta is the result.
-Paraphrase of Khensur Rinpoche Lobsang Tsephel citing the Guhyasamaja Tantra
"All memories and thoughts are the union of emptiness and knowing, the Mind.
Without attachment, self-liberating, like a snake in a knot.
Through the qualities of meditating in that way,
Mental obscurations are purified and the dharmakaya is attained."
-Ra Lotsawa, All-pervading Melodious Drumbeats