When making a stencil the "impossible" trick is to create an island, a blocked out section of the stencil that isn't connected to the side. I wanted to apply an image only once so I made this technique to have easy and clean islands. This works for applying fabric paint. For spraypainting there are other methods that can be used, even reusable ones.
Step 1: Put Together a Design
Digging back into the old-school video games is always good for some easy stencils. It's all right angles with the pixels blown up. Here's a version of one of the alien ships from Space Invaders. The two eyes are some easy islands to deal with, although I've used this technique for smaller and curvier pieces as well. Be sure to use the Find Edges filter so you don't use up as much toner.