Note: your target needs to have a black or very dark vehicle for this to work.
Step 1: The secret sauce
Small pieces can be easily found on eBay. I got a 6" x 18" piece for about $10, including shipping. That's enough for 4-6 bumper stickers depending on how tall you want them.
eBay search for reflective tape
Step 2: Make a message and cut it up
OK, you have your bit of funny. What you want to do is block out where you want your message to appear so that you can spray paint the rest of the sticker.
For me, I had it easy because we have an Epilog laser cutter. I made two overlapping pieces so that I quickly spraypaint it in two passes.
For most everyone else, print out your message on paper and cut it most of the way out. Leave little bits so that it's still attached.
Step 3: Get sticky with it
Spray the backs of the stencils with a light coat.