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.
Step 2: Print and Get Ready to Cut
Since this is being done as a one-off you can use regular printer paper or even reuse a piece of paper from elsewhere. Just as long as you can print on it.
After that, just get out your Xacto knife and get ready to start cutting along the lines.
Step 3: Cut Most of It
Cut most of the lines, but leave a few lines unfinished. The effect we're going for here is that all these pieces are still together. And where the lines are unfinished, LIGHTLY score them with the blade.
If you look at the second picture, you can see that the eye is now a barely on window flap.
Step 4: Spray It and Stick It
Get out some spray mount and get a light layer of the stuff on the back. Or the front since this design can be flipped. Whatever makes you feel happy right now. Wait a minute or two and then stick it down, pressing all around to make sure there's contact.
Step 5: Peel
This part's pretty self-explanatory. Peel off the middle chunk so that it becomes a stencil. When you get close to the islands in the design, hold them down with one finger while you carefully peel off the rest with your other hand.
You might get tiny pieces sticking off of the islands like in the third picture here. If that happens just get the Xacto out again and lightly scrape it off.
Step 6: Paint the Sucker
Get out some fabric paint and a brush and go to work. You know what to do here. Lay it on thick for a solid color or go for a lighter "sketchier" approach like this one.
Step 7: Peel Again
Now it's time to peel off the stencil. The main part should come off without a problem. As for the islands, use some tweezers to pinch at the side of one and then peel it off. Avoid using some pokey object to shove in from the side to lift it off. You can end up shoving some paint underneath and mess it all up, especially if you did a thicker layer of paint than the thin coat I did here.
Step 8: Finito!
Sit back and relax, knowing that you've done your bit to further solidify an ancient game into the absorbent fabric of pop culture. Heat set if you need to.