Stencil Revolution has a solid set of instructions for stenciling your very own shirts. Of course, it helps if you're familiar with how to make a stencil, but that should be pretty apparent. Just let your inner Martha Stewart out so she can do her thing and you ought to be fine.
P.S. If you want to save yourself some heartache, make a tester shirt or two on some inside out shirts from the goodwill. Never just cut a stencil and think the first shirt you do is going to turn out perfectly!
MATERIALS: I use acetate stencils. Other stencils will work as well. They can't be too thick though. Fabric paint seems to do the trick. It goes on nice and thick. I keep a stock of primary colours that make it easy to mix up. For white I have a pot of supercover white, which is a lot thicker but will go on black no problem. It takes a long time to get through a small pot. I've found this brand (Permaset) Fabric paint at most 'good' art supply stores. Foam roller. You can usually get cheap ones from craft stores, I go through these pretty quickly so it's best to buy heaps of cheap ones than splash out and try and buy one that will last. The paint just eats away the foam after a while. Sometimes you get a free little paint tray which is also useful. Masking tape. Good for everything! Lay your stencil down and make sure the fabric is flat. Tape the stencil where ever you can. Mask off seems and stuff like that. Load your roller up, you don't need to much paint. The trick is to do more small coats than one sloppy one. You acquire a feel for rolling on fabric and you can begin to apply pressure. Make sure you have an even coat. Be careful when ripping of stencil and tape. Shirts will dry to be touch dry if you blast them with a hairdryer for a few seconds, so you can flip it over and print on the back. To heat seal a shirt, put a piece of baking paper on the printed area and iron for a few minutes, just enough to heat it up and melt it a little. You can't really burn it unless you walk away with the iron cooking your shirt. If you are doing multiple layers I recommend heat sealing in between each colour. Now whack a tag on it and sell it in a store, give it away as a present or print up your very own Che Guevara shirt!