To make the stencils I just printed variations of the design on full page sticker paper, on an ink jet printer (laser jet printers have trouble with the thick paper). Then I cut out the design with an exacto knife, so essentially I end up with a stencil that is also a sticker. This means it can stick directly to the fabric and doesn't move around while painting. However, it also means it's not reusable, so I would only suggest this method for simple designs and only a few prints. I was going a little bit crazy by the time I was cutting out the 6th stencil!
I went to the Downtown Rescue Mission Thrift Store, which is by far my favorite thrift store in Huntsville. They have a huge selection of clothes, and if you pay attention to which tags are discounted you can get stuff for really cheap; the T-shirts I bought ended up averaging to about a dollar each.
The yellow one is my new biking shirt:
The paint got a little messed up on this one, and I later I noticed the design was crooked too, so at least I got all the screw ups out of the way on one shirt!
For the gray one I made the stencil a lot smaller and put it where the pocket would be if there was a pocket.
For the green shirt I left out the Let's part and just printed the GO bicycle larger.
After the last shirt I decided I really liked the GO design by itself, so I made two small stencils to go on some old pairs of shorts I already had.