1. Figure out your collage and arrange the pieces on the denim to see how it looks.
2. Smear the back of one piece of paper in the collage with clear silicone (available at any hardware store; I use Dow Corning Auto Marine Sealant). Affix that piece in place on the denim. Repeat with all collage pieces. Let dry.
3. Use a large piece of tracing paper, lay it over your collage, and trace each collage piece, allowing about a quarter-inch extra margin around each piece. Carefully cut the shapes you've drawn out of the tracing paper. Cut only the shapes themselves. The end result should be a solid sheet of tracing paper with holes that mirror your collage pattern.
4. You are now going to use the tracing paper as a mask for the final glue layer. Place the tracing paper on the collage again, matching the holes in the tracing paper to the shapes in the collage. You may tack the tracing paper down here and there using a little glue stick (which is water soluble) if you'd like, to make the next step less nerve wracking.
5. Using your fingers (with paper towels nearby for emergencies), smear silicone over each hole in your tracing paper. This should create a silicone seal over each shape in your collage, and that seal should extend about a quarter-inch around each collage image. You can see how the seal extends beyond the image in the last picture above. When all shapes have been siliconed, lift the tracing paper away. Use a toothpick or q-tip to neaten up the edges of the shapes. Let dry.
The end result is a collage that is silicone-sealed on front and back, making the paper impervious to moisture, dirt, or whatever.
I created this "let them eat stones" collage during my stint as a restaurant reviewer (10 years for the East Bay Express; 2 years for SF, the Magazine of Design and Style; and several editions of Fodor's guides to San Francisco). The denim jacket is sort of a James Dean approach to eating for a living in a town full of foodies.