Supplies you need: old/new/used canvas or wood panel; Sticker letters (I found mine at Hobby Lobby in the stencil aisle), spray paint, X-acto knife.
STEP ONE: Prep your surface. I painted over the original painting with bright geometric shapes at random. It makes for a fun surprise in the end! Cut our your letters and play with spacing on your surface until you come up with a composition you love.
STEP TWO: Place your letters ... make sure you don't have any air bubbles and that all letters are pressed down completely to avoid any paint bleeding under.
STEP THREE AND FOUR: Spray paint your entire surface and let dry for at least an hour. Then carefully (I suggest using an X-acto knife) begin peeling off your letters.
STEP FIVE: Enjoy your new piece!
*** Note: I did run into a sticky residue issue after I peeled the letters off. However, if you rub the surface with your fingers it comes off sorta of like rubber cement ***
I have been experimenting with this technique some on other projects as well, I definitely be showing them once they are complete... have fun creating!