When you practice the breathing techniques of Wim Hof, and follow the Wim Hof courses, you can eventually control your autonomic nervous system, and boost your health. But, there also are some calming breathing techniques that you can easily do yourself. The only thing you need is just a few minutes of leisure time.
If possible, loosen any clothes that restrict you from breathing freely. To start, breathe in through your nose, and out through your mouth (lips pursed, as if you are whistling). Let the breath flow as deep as is comfortable into your belly. Breathe regularly and gently. If you are too distracted to focus your mind, steadily count from 1 to 5 during both inhaling and exhaling.
It is important that you practice these exercises regularly, for example as a part of your daily routine. Each time, do these exercises for about 5 to 10 minutes. Before you start, make yourself comfortable. The exercises can be done standing up, lying down or sitting. As long as you are comfortable and steady.