You will need a foam block, the stencil template from the Cover the Night Digital Action Kit, some tape that will stick to both, and a knife. Scissors may also be helpful. The foam block I used came from some packaging. My parents tried to throw it out, but I saved it because I knew I could find a use for it. It happens to have three holes in the back that make a passable handle.
Warning: Don't be stupid like me and make your stamp the same way around as the stencil! Make sure you flip the template horizontally before/when you print it. I'm going to use the extra foam I cut off in step 3 to redo the text. Also, if you use a molded surface of closed-cell foam, you'll probably have to roughen it for the chalk to stick.