One goes to the forest, to the foot of a tree, or to an empty room, sits down cross-legged, holds one's body straight, and establishes mindfulness in front of oneself.   Breathing in, one is aware of breathing in.
Breathing out, one is aware of breathing out.

Breathing in a long breath, one knows, 'I am breathing in a long breath.'
Breathing out a long breath, one knows, 'I am breathing out a long breath.

Breathing in a short breath, one knows, 'I am breathing in a short breath.'
Breathing out a short breath, one knows, 'I am breathing out a short breath. The Buddha