I believe Crayola sells scratch paper now, but it was more fun to make our own, plus Tyler got to see and understand how it works. One day as he was using crayons, I took out my own paper and filled it completely with random blocks of color. You can make stripes, designs or whatever seems fun, but I went with what I tended to do as a kid. Older kids can of course do this themselves.
The next step is to color it over hard with black crayon. When I was done, I asked Tyler if he wanted to help a bit. He had been watching me and was quite eager!
Later, I gave Tyler some colored pencils with sharpish broken tips to use to scratch away the black crayon with.
It was difficult for him to apply enough pressure to really scratch away the black on his own, so we tried using other objects as scratching tools, including his scissors and his toddler knife (both way too blunt).
He didn't mind me helping him push down as he drew, but it wasn't ideal. The results were fun for him to see!
After his work had been hanging on his cork board for a few days, Ty came over and scratched at it with his nails. I knew he was due for a trim at this point, but it worked! He was able to scratch away a good deal and was quite pleased!