The late Shunryo Suzuki 鈴木 俊隆, founder of the Zen Center of San Francisco, offers these instructions on breath meditation:
When we practice [meditation] our mind always follows our breathing. When we inhale, the air comes into the inner world. When we exhale, the air goes out to the outer world. The inner world is limitless, and the outer world is also limitless. We say ‘inner world’ or ‘outer world’, but actually there is just one whole world. In this limitless world, our throat is like a swinging door. The air comes in and goes out like someone passing through a swinging door.
If you think, ‘I breathe’, the ‘I’ is extra. There is no you to say ‘I’. What we call ‘I’ is just a swinging door which moves when we inhale and when we exhale. It just moves; that is all. When your mind is pure and calm enough to follow this movement, there is nothing: no ‘I’, no world, no mind nor body; just a swinging door.
source: Suzuki, S. (1973). Zen mind, beginner’s mind: informal talks on Zen meditation and practice. Weatherhill. This quote is actually from the 2011 edition, p. 11.