Lie down on your back. Put your hands on your stomach. As you breathe in, your hands should expand and rise with your stomach. When you exhale, your hand should return to the resting position.
Inhale and Exhale to the count of 5 (increase this number as you get better at it) with average lung capacity inhaling and exhaling to a slow count of 10 is good! Focus on your inhaling and exhaling, watching your hands rise and fall.

Variations:
Make noise on inhaling, pucker your lips and suck the air in.
Inhale through your nose, keep your mouth completely closed
Make noise on the exhaling or whistle as the air goes out.
Put your hands behind your head, elbows out to the sides, this will expand your lungs even more!
Add weights to your stomach.