Breath awareness is the most fundamental and widely practiced form of meditation. The instructions are simple: sit comfortably, close your eyes, and place your full attention on the physical sensation of breathing. Notice the breath entering and leaving the nostrils, the rise and fall of the chest, the pause between breaths. When your mind wanders — and it will — gently return your attention to the breath without self-criticism. This returning of attention is the actual practice. Each time you notice you've wandered and come back, you are training the mind's capacity for attention and self-awareness.

Reference:

Wikipedia: Anapanasati (Breath Meditation)

https://en.wikipedia.org/wiki/Anapanasati