When beginning to learn to breath using your diaphragm, it is a good idea to lay down on your back with a pillow under your knees, this helps to encourage your body in to a state of relaxation. Think about where you are holding tension in your body. Let it go.
When practicing this breathing, it is important to breath in through your nose. You want to pretend that the opening of your nose is in the back of your throat, not in your nostrils. Be patient with yourself, this kind of breathing takes some practice.
Without straining your breath you will want to direct your breathing to your belly. Your belly should blow up like a balloon.
Let your breath go out of your body with a whoosh. It will do you good to hear the sound of your breath leaving your body.
Repeat 5 times, 2 times a day, in the morning, and while laying in bed before going to sleep.
This is a good exercise for when you find that you are feeling stress throughout the day as well.