Select a clip art (or other simple image) to use for stencil. Enlarge and print on to an 8 by 12 inch piece of white paper.
Tape your image down on the cutting board. Then place the mylar stencil film on top of your image and tape it into place.
Use the marker to trace the outline of your clip art on to the mylar. Once you have completely traced the outline you can remove the print of your clip art and tape the mylar to the cutting board again (or, if you don't want to reuse the printed clip art, you can skip to the next step).
Carefully use the exacto knife to cut out the interior area of the outline. Remember that you are cutting out the parts of the image that will contain the paint color! Your stencil will provide the outline of the image that you want to paint on your wall.
Remove your new stencil from the cutting board and lay flat on top of the plastic drop cloth (or a flattened, large plastic bag). Spray with repositionable adhesive spray (follow the manufacturer's directions).