When you do kapalbhati, you forcefully contract your stomach on exhalation and immediately relax the muscles completely. You can imagine that the air fills you out like a rubber toy with a vacuum inside, from which the air was just squeezed out. The air inflow should be totally passive. The breathing takes place through the nose only.
It should not be attempted by people who suffer from:
Because this is highly energizing practice, Kapalbhati is best performed before the asanas or in the morning. In the evening, it may difficult to fall asleep. You should do this pranayama on an empty stomach, with at least 2 to 3 hours break after eating.