Mindfulness of breathing (ānāpānasati) is one of the most complete meditation systems in the early discourses. In MN 118, the Buddha says that when it is developed and cultivated, it fulfills the four foundations of mindfulness, the seven awakening factors, and knowledge and liberation. It is not presented as a relaxation trick, though calm is part of its development.
The setup is plain: go to a quiet place, sit with the body upright, establish mindfulness, and know breathing in and breathing out. The first four trainings concern the body: knowing long and short breath, experiencing the whole body of breath, and calming bodily activity. One does not need to force the breath into a special rhythm. The basic act is intimate knowing. If control creates strain, return to receiving one natural breath from beginning to end.
The next four trainings concern feeling: experiencing rapture and happiness, experiencing mental activity, and calming that activity. The next four concern mind: experiencing, gladdening, collecting, and liberating the mind. The final four concern Dhamma: contemplating impermanence, fading of passion, cessation, and relinquishment. The sixteen steps show a movement from simple presence toward insight and release.
Traditions teach the details differently. Some attend at the nostrils, others to the abdomen or whole-body experience; some emphasize counting at the beginning; some integrate the sixteen steps explicitly. A course can explain these maps, but sustained practice is best learned with a qualified teacher who can respond to dullness, agitation, fear, unusual bodily experiences, or attachment to pleasant concentration.
For a beginner, ten minutes is enough to learn the posture of practice: recollect refuge and precepts, sit steadily, know where the breath is clearest, recognize wandering without anger, and return. Success is not a blank mind. Each honest recognition of wandering is mindfulness re-established.
On a full-moon observance, the Buddha surveys a Saṅgha containing practitioners at many stages and names multiple meditation themes. He then presents breathing as a practice that can fulfill the entire sequence from mindfulness to liberation.
The Buddha teaches Rāhula earth-like patience, the elements, loving-kindness, compassion, impermanence, and finally mindfulness of breathing. The instruction is embedded in ethics, emotional balance, and wisdom.