The Fukanzazengi by Dogen Zenji is good instruction, but is very difficult to understand. It is especially hard to comprehend how to work with the mind, and how the practice relates to enlightenment. I will briefly explain how to practice shikantaza.
Generally speaking, zazen can be described in three phases: first, adjusting the body, second, the breathing, and third, the mind. The first and second are the same both in koan Zen and shikantaza. However, the third, adjusting the mind, is done very differently in the two practices.
To do shikantaza, one must have a firm faith in the fact that all beings are fundamentally Buddhas. Dogen Zenji says in the ninth chapter of Precautions on learning the Way:
You should practice along with the Way. Those who believe in the Buddha way must believe in the fact that their own self is in the midst of the Way from the beginning, so that there is not confusion, no delusion, no distorted viewpoint, no increase or decrease, and no errors. To have such faith and to understand such a way and practice in accordance with it is the very fundamental aspect of learning of the Way. You try to cut off the root of consciousness by sitting. Eight, even nine out of ten will be able to see the Way- have kensho- suddenly.
This is the key to practicing shikantaza. But this does not at all mean that one must believe that one's small-minded, self-centered life is Buddha's life-on the contrary! Casting all sorts of self-centeredness away and making yourself as a clean sheet of paper; sit, just firmly sit. Sit unconditionally, knowing that sitting itself is the actualization of buddha-hood- this is the foundation of shikantaza. If one's faith in that fact is shaky, one's shikantaza is also shaky.
In doing shikantaza you must maintain mental alertness, which is of particular importance to beginners-and even those who have been practicing ten years could still be called beginners! Often due to weak concentration, one becomes self-conscious or falls into a sort of trance or ecstatic state of mind. Such practice might be useful to relax yourself, but it will never lead to enlightenment and is not the practice of the Buddha Way.
When you thoroughly practice shikantaza you will sweat-even in the winter. Such intensely heightened alertness of mind cannot be maintained for long periods of time. You might think that you can maintain it for longer, but this state will naturally loosen.
So sit half an hour to an hour, then stand up and do a period of kinhin, walking meditation.
During kinhin, relax the mind a little. Refresh yourself. Then sit down and continue shikantaza.
To do shikantaza does not mean to become without thoughts, yet, doing shikantaza, do not let your mind wander. Do not even contemplate enlightenment or becoming Buddha. As soon as such thoughts arise, you have stopped doing shikantaza. Dogen says very clearly, "Do not attempt to become Buddha."
Sit with such intensely heightened concentration, patience, and alertness that if someone were to touch you while you are sitting, there would be an electrical spark!
Sitting thus, you return naturally to the original Buddha, the very nature of your being.
Then, almost anything can plunge you into the sudden realization that all beings are originally buddhas and all existence is perfect from the beginning. Experiencing this is called enlightenment. Personally experiencing this is as vivid as an explosion; regardless of how well you know the theory of explosions, only an actual explosion will do anything.
In the same manner, no matter how much you know about enlightenment, until you actually experience it, you will not be intimately aware of yourself as Buddha.
In short, shikantaza is the actual practice of buddhahood itself from the very beginning-and, in diligently practicing shikantaza, when the time comes, one will realize that very fact.
However, to practice in this manner can require a long time to attain enlightenment, and such practice should never be discontinued until one fully realizes enlightenment.
Even after attaining great enlightenment and even if one becomes a roshi, one must continue to do shikantaza forever, simply because shikantaza is the actualization of enlightenment itself.
Hakuun Yasutani (1885-1973)