‘Correct breathing is important for zazen, for one must achieve harmony of mind and breath. Taiso-Daishi believed that certain bad mental states were caused by lack of harmony between these two. Before zazen, a few very deep breaths help quiet the mind.
Then after swinging one’s body from side to side in order to achieve balance, one should again breathe deeply from the abdomen for a few minutes before returning to normal breathing. This normal breathing should be silent and not rough. It is important not to inhale too much air for a long period, as it is tiring.
In the Rinzai-school, strong pressure on the abdomen (while breathing) is perhaps over-emphasized. It is not right to cling to the idea of breathing in a special way.
A few deep breaths should establish the center of gravity in the lower part of the body. First, inhale slowly through the nose, using the air to expand the abdomen as far as possible; then exhale slowly (through the nose) till the abdomen has again contracted. It is necessary that the center of balance be in the abdomen. Chikara, which means force or power in the abdomen, is essential in all traditional Japanese physical culture.
I will now speak of the proper functioning of the mind during zazen. Beginners often ask me about their problems; however, it is very difficult for me to be of any help to them. Neither a short nor a complicated answer to peoples’ questions is really helpful. It is all right to ask me questions, but it is not enough. One must experiment for oneself and then one will understand. After reading a book on the subject of swimming one must get in the water and find out about it first hand. A book cannot give one the experience. (The Way of Zazen)
And this, which was one of the only instructions available in English in the 1960s, has not dated one little bit.