I've spray paint stenciled clothes for a while, and have a few shirts that I spray painted about 7 years ago that still hold strong. After a good hanging out to get the fumes out and a run through a washer, it's all ready to wear.
I'm planning on doing some more thorough experiments and will share notes as I go.
Step 1: Make a stencil...
The trick is to go very light with the first spray you do. If you spray a thin even coat on your paper, and let it dry, then repeat, ie, build up slowly, you get a waterproof coating on your stencil paper. If you spray too hard you can soak or sog it down and it either gunks up or rips and falls apart.
I made some stencils on a whim after pulling a cardboard donut out of a 2gal glass bottle I bought and thinking it would be a nice frame for a stencil.
The other bits are just duct tape and printer paper cut after being folder over 2wice and cut out with scissors (I reinforced their bottom with ducttape).