1) Cut stencil out of thick card stock or plastic (for sale or beware of dog signs work great), using x-acto knife. Use cutting board for safety.
2) Hold or place flat on surface. For best results tape or use spray adhesive.
3) Apply chalk to cut out areas to transfer design and fill with chalk.
4) The stencils can be used with spray paint. (using spray paint may result in prosecution, Spray at your own risk.)
For poster making, get your image ready in black and white then blow it up with a copier or projector as large as you desire, then reproduce the design on xerox. You can also try silk screening or printing but it's way more expensive. Silk screening requires screens, photo emulsion and ink. Offset printing is pretty convenient for runs over 500 units but it's also expensive. For all of you interested in making and pasting your own posters, the easiest way to paste is to buy wheat paste (try All in Onebrand) also called wall paper paste at your nearest hardware store it will cost around $2.00 - $5.00 a box, you'll need a big brush (4? or 6?) and a bucket, soak surface, place the poster then brush the poster hard enough to avoid bubbles.