Quite simple. Don't grasp whatever occurs in the mind - the complete field of experience - but just let it come and let it go. Then you add to this all the formalities of a Soto Zen temple, if you want to.
In ''let it come and let it go'' is a danger of observing, which is not shikan taza. It can turn into just idle sitting, after all. ST is powerful position of direct entering into enlightenment in one leap so to say. For this one needs clear guidance of deeply realized person. Since it is most simple, has deepest dangers of wrong ideas. ''Direct'' and ''entering'' are both misleading as well, since it is beyond any conditions or extremes, if one uses conventional language. So it may be the point which is difficult for the mind habituated by conditions, including ''let it'' and so on.
The other very difficult point would be no-thinking etc. because it has nothing to do with thinking as we think about thinking and no-thinking as two extremes... it is free from that. But here is point for the ''mouth transmission'' or KUDEN - key instructions, [but they are not written, however one is allowed to make notes] of one's own master who should have great skills.
There is more to it. On extraordinary level of preparation one is required to manifest genuine bodhicitta as taught by Tendo Nyojo and Dogen, who followed Nagarjuna teaching in this respect (motivation) and powerful amount of faith or confidence (goal), which for ordinary individuals is obscured basically by karmic obscurations or habits... In this case then it comes down to important rituals, maybe those are ''soto formalities'' which include very basic and regular practice of purification there are 3 of them, and important part for the initiated into this way is the blessing of the personal lineage which is practiced by oneself, or individually. Not in the group. Again at that point one should rely on KUDEN for purification, they are different levels of understanding and blessing, which is most intimate.
Some points, but not all, one can find in Dogen's GAKUDO YOJIN SHU, and its important commentaries. As for commentaries I am afraid there are none in English or any Western language. The original text of Dogen is translated by someone I guess.