This is a foundational exercise that is common to most programs in mindfulness training. You start by sitting in a comfortable upright posture and focusing all of your attention on the sensations of breathing.
Select a particular sensation that is tied to your breathing, such as the coolness of air moving in and out of your nostrils or your abdomen moving up and down.
Maintain your attention on that sensation for the period of time you’re going to engage in the practice—for example, 10 to 15 minutes.
Notice when your mind wanders away from the sensations of breathing. When you notice that it has wandered, gently return your attention to the sensations of breathing.
These three main components—select, maintain, and notice—are the central driving hypotheses of how we think mindfulness training may be altering the brain. Does mindfulness training allowing for process‑ specific training of attention?
Resource: Masters of Mindfulness: Transforming Your Mind and Body