The Buddha discusses mindfulness of breathing here:
MN118:15.1: Mendicants, when mindfulness of breathing is developed and cultivated it is very fruitful and beneficial.
Find a comfortable, quiet place:
MN118:16.1: And how is mindfulness of breathing developed and cultivated to be very fruitful and beneficial?
MN118:17.1: It’s when a mendicant has gone to a wilderness, or to the root of a tree, or to an empty hut. They sit down cross-legged, with their body straight, and establish mindfulness right there.
Westerners normally have pain sitting cross-legged, so you may need a cushion or even a chair to support your body comfortably. A well-supported body is comfortable.
The body should be straight simply because that is comfortable for a long time. A hunched over posture leads to pains in the neck and back. Meditation is also not a military drill where a drill instructor whips out a micrometer to check you against some ideal. The posture should be tranquil:
MN118:35.1: When the body is tranquil and they feel bliss, the mind becomes immersed in samādhi.
Finding a tranquil posture, gently observe your breaths. Note how the breath flows as you breath. And this is important. We observe the breath. We do not struggle with the breath.
MN118:17.2: Just mindful, they breathe in. Mindful, they breathe out.
MN118:18.1: When breathing in heavily they know: ‘I’m breathing in heavily.’ When breathing out heavily they know: ‘I’m breathing out heavily.’
MN118:18.2: When breathing in lightly they know: ‘I’m breathing in lightly.’ When breathing out lightly they know: ‘I’m breathing out lightly.’
As you meditate mindful of the breath, you may notice that breathing slows and deepens to fill more of the lung. This is natural and does not require struggling. Just peacefully meditating, the chest opens up, the shoulders relax, the chin tucks in, the back straightens, the diaphragm moves the breath.
This sounds simple but isn't. The mind races about frantically asking, "is this right? is this working? is this bad? how long do I have to keep doing this?" And that babble of worrisome thoughts feels like a flood. Let the flood go and attend to your breath. A very effective way to attend to your breath is to count your breaths. Counting our breaths is challenging but manageable. If you lose count, start again or keep going. Just count your breaths. Counting is important because counting requires that we note each breath we take.
As mindfulness of breathing increases and deepens, we become aware of breathing throughout the day. We start to notice subtle things. Chopping vegetables we may discover that breathing mindfully can help us from cutting ourselves. Driving our car, we may notice that breathing mindfully stills road rage. Feeling depressed and inadequate, we may find that our gentle breath keeps going on, sustaining and nurturing us through our darkest moments.
May you find peace in your meditation, in your breathing, in all that you do.