Diaphragmatic breathing signals to the body that the threat has passed and it is safe to relax. It is slower and deeper than normal breathing, and happens lower in the body (in the belly or abdomen). You should practise this breathing technique three times a day or whenever you feel anxious. It can also be used to slow breathing down when hyperventilating (rapid breathing).
Listen to this audio which will talk you through the technique: Diaphragmatic or abdominal breathing (2 mins) or watch the MindWell Diaphragmatic breathing animation video.