About this passageThe discourse is set on a full-moon night at the end of a rains retreat, and most of it is the Buddha describing the community sitting in front of him. These lines are the method itself, the first two sets of four out of sixteen. What is asked for is unusually small: notice whether the breath is long or short. Nothing is to be changed, counted or achieved; the later steps do not ask for a different breath but for a different quality of noticing while it goes on by itself.
It's when a mendicant—gone to a wilderness, or to the root of a tree, or to an empty hut—sits down cross-legged, sets their body straight, and brings mindfulness to the present. Just mindful, they breathe in. Mindful, they breathe out.
Breathing in heavily they know: "I'm breathing in heavily." Breathing out heavily they know: "I'm breathing out heavily." Breathing in lightly they know: "I'm breathing in lightly." Breathing out lightly they know: "I'm breathing out lightly."
They practice like this: "I'll breathe in experiencing the whole body." They practice like this: "I'll breathe out experiencing the whole body." They practice like this: "I'll breathe in stilling the physical process." They practice like this: "I'll breathe out stilling the physical process."
They practice like this: "I'll breathe in experiencing rapture." They practice like this: "I'll breathe out experiencing rapture." They practice like this: "I'll breathe in experiencing bliss." They practice like this: "I'll breathe out experiencing bliss."
They practice like this: "I'll breathe in experiencing the mental process." They practice like this: "I'll breathe out experiencing the mental process." They practice like this: "I'll breathe in stilling the mental process." They practice like this: "I'll breathe out stilling the mental process."
In plainer words
Go somewhere quiet — outdoors, under a tree, an empty room. Sit down, keep your back straight, and bring your attention to what is happening now. Then just breathe in and out, and know that you are. When the breath is long, know it is long. When it is short, know it is short. Then: breathing in, take in the whole of the body; breathing out, the same. Breathing in, let the body settle; breathing out, let it settle. Then the same again with gladness, with ease, with whatever the mind is doing, and with letting the mind settle.
A restatement in modern English, written for this collection. The passage above is the text; this is one way of putting it.