1. Place a piece of cardboard into bag to keep the paint from bleeding through.
2. Tape the stencil down (I use painters tape.) on the bag and, using a spouncer, apply paint to stencil.
4. Allow to dry and then fill with your favorite candy!
I also created an elegant, Halloween door knob hanger for the Paint Me Plaid blog using a similar technique. You can get the tutorial here.