Scratch art is very easy to do. Just make a first layer of colors. Next cover that layer with black crayon. Then use a pointy object like a toothpick or end of a paper clip to scratch into the top layer so that the bottom layer of color appears.
Scratch art helps children learn how much force to use because if they press too hard the bottom layer of color will be scratched away and if they do not press hard enough the top layer of black will not be removed. This is a fun way to controlling the writing tool with the tripod fingers (index, middle fingers and thumb) since the child really needs to control that tip in order to scratch a picture. At the same time there are no demands to form letters so the focus is on controlling the writing tool and having fun. Developing pencil control prepares children to form letters.