To make the scratch board paint, I usually just squeeze 1-2 tablespoons of dish soap into an almost full container of paint. I'm sure there is an actual recipe out there somewhere, but I figure once the paint starts smelling like the dish soap, there's probably enough in it to make a difference. You'll notice that the paint will be a little bubblier from the soap once you shake it up and a little shinier than normal during application.
Next, paint over the crayon with the scratch board paint! I like using the sponge brushes. I usually have the kids do two coats. Be sure to use long, even strokes across the paper. If your paint ends up being too think in spots, it will end up being difficult to scratch precise details because the paint will want to chip off. Paint a single layer, let dry, and then paint another. Today I had the kids take their papers to the heater and hold it upside down over the air to dry their layers faster. Everyone was able to prepare their scratch boards in one class today.
These scratch boards are for the 6th grade Greek unit. Students are currently learning about the evolution of Greek pottery, starting with Minoans and the geometric style. They were asked to choose a type of Greek pot to represent in scratch art, thus the black and red colors. Some students are doing red crayon with black paint while others are doing black crayons with the red paint to replicate the idea of red-figure vs. black-figure pottery.