Freezer Paper Stenciling
You're going to need... Freezer Paper!!! HA! You can buy it in the aisle with aluminum foil and waxed paper. You get a HUGE roll that will last forever for about four bucks. There are a lot of things you can do with this if Pinterest is to be believed. ;)
You're also going to need to print out your design. I went with the logo that the main character on a popular Mexican tv show from when I was a kid wore on his chest. Cut it out with an X-Acto knife. Alternately, you could cut this out with a Silhouette/Cricut. Just be sure that you stick the paper to the cutting mat shiny side down.
Iron the piece of freezer paper to your shirt. The iron melts the plastic coating on the paper just enough to get it to stick to the fabric, but not enough to ruin it.
Put a piece of paper/cardstock inside the shirt to protect it from the paint bleeding through, then just paint inside your stencil with fabric paint! I did learn that not all fabric paint is created equal. I had a bunch of puff paint so I figured I'd just use that since the color I wanted was not in my collection in regular fabric paint. Bad move - it took about fifty coats because it ended up being way sheer when brushed out! Lame, but eventually I got good enough coverage.