With your hands across your abdomen, using your stomach(diaphragm) to inhale/exhale and try the 7 11 breathing technique:

inhale for 7 seconds
exhale for 11 seconds

if thats too long try 5 & 9 seconds and eventually work your way up to it and go higher if you can but dontstrain yourself.

works by using your para-sympaphetic nervous system to slow your heart, and by placing your hands on your diaphragm just helps to focus on your breathing.