Locate a poster-sized representation of the image you wish to create a stencil of, or create the image yourself on a poster-sized piece of paper. You can use any image you wish to create your stencil. You'll cut into the image beneath the sheet during the stencil creation process, so choose one that's disposable.
Tape the picture that you wish to use to create your stencil to a piece of clear acetate sheeting large enough to cover the image. Tape the entire perimeter of the paper to the acetate with the masking tape, making sure that there are no wrinkles along the paper's surface. Purchase poster-sized sheets from an art or hobby supply store. The acetate sheet is clear enough to trace the cutting lines you need from the image beneath while being thin enough for cutting with a hobby knife and strong enough for multiple uses as a stencil.
Place the taped paper and acetate onto a cutting board. Tape the combined sheet to the board, completely around the perimeter of the sheet, to prevent its moving while cutting out the stencil.
Cut out the lines for your stencil from the acetate sheeting with the hobby knife using the image taped to the sheet as a guide. Cut lines or sections in the sheet that you want the paint to pass through to the surface to which you're applying the stencil.
Remove the acetate from the cutting board and the image. Use the stencil by spraying the rear with a repositionable spray adhesive from a distance of about one foot away. Allow the adhesive 15 minutes of drying time, then spray again. Wait a second 15 minutes and press the stencil onto any flat surface. Spray paint over the front of the stencil so that the paint passes through the cut lines to apply your poster-sized stencil image to the surface.