Start by taking a normal breath, counting the seconds it takes you to inhale through your nose. Exhale, through pursed lips, for twice the number of seconds as you inhaled. Try to make a slight whistling noise as you exhale. For example, if you inhaled for three seconds, exhale for six seconds. Don't force the air out, just breathe out in a controlled manner, through your lips.
Abdominal breathing is another great breathing technique.
Lie on your back with your knees bent or place a pillow under your knees. Put one hand on your upper chest. Place the other in the center of your abdomen, at the base of your breastbone. Exhale slowly through pursed lips, squeezing your abdominal muscles upward and inward. You will feel your abdomen pressing towards the floor.