First you can practice this technique by sitting or lying down peacefully.
Afterwards you will be able to practice this technique in any given time or location-while you are pacing, at your work place or even riding the bus.
At the start of the first inhalation, mentally label it “in one”. When you begin to exhale, label that as out one.
As you begin to breathe in again, all in two and then as you start to exhale, call it out two. Keep doing it until get to four:
“In one…Out one
In two…Out two
In three…Out three
In four…Out four”
Then begin at one again. Completing four whole breaths are about as many as beginners can keep a record of with smooth focus.
You might discover you become easily distracted and lose focus of your current location. You might even say, “This is silly. Why am I wasting my precious time?” If that happens, just regroup and focus your mind and start again at one.
Because this form of exercise is basic and unobtrusive, you can practice it in crowded, loud locations.
Source: Mckay, K., Davis M. & Fanning, P. (2007). Thoughts & Feelings: Taking control of your moods & your life. (3rd ed.). Canada: New Harbinger Publications, inc.