So I read about how to do this online a while ago, pretty sweet if you get the hang of it.
What you do is fill your mouth with air from your lungs then compress that air in your mouth as hard as possile for about 10 or more seconds. Relax your mouth with it still closed then slowly push the air out of your mouth with your tongue. You should be able to see steam. If it doesn't work, try it in different lighting.