Step 1: Supplies
1 crayola dry erase marker (black)
This crayola dry-erase marker is awesome. It pumps out a really bold black line better than others I've tried.
Step 2: Coat the transparency in dry-erase ink
Step 3: Scratch your design in
Use some sort of blunt tool to scratch your design onto it. I used the wrong end of a paintbrush, but you can use almost anything. I've also had success using a pencil eraser to remove the dry-erase ink. A clay modeling tool works too.
If you make a mistake, you can just color back in using the dry-erase marker, let the ink dry and re-do it.
Be careful not to rest your hand or fingers on the drawing, or the dry-erase ink will come up off of it and leave a nice big palm print on your artwork.
You might want to leave some margin of non-drawing around the edges so you can handle the thing easier, without getting fingerprints in it.
Step 4: Put something colorful underneath
I used my cheap watercolor block with some random paint on it.
Step 5: Scan it in
In other words, create a two-layer sandwich with the ink facing up. This means you can re-use the colored background, and wipe and re-use the transparency as needed. [thanks Canida for the clarifying words].
Be sure to wipe away any dry erase ink that may get onto your scanner surface when you're done.
This is a very inexpensive way to create scratchboard art. Have fun and be fearless.