Measure, mark and cut the 3/4-inch MDF into a circle with a 24-inch diameter. Many hardware stores sell round MDF, but if you can't find it, purchase a 24-inch by 24-inch square MDF and mark the circle by using a string attached to a thumbtack as a compass. Push the thumbtack in the center of the circle; tie one end of the string around the thumbtack and the other end around the marker, for a radius of 12 inches.
Draw a second 2-inch to 3-inch circle by shortening the string; if your string is 1 inch long, your circle will have a 2-inch diameter. The center circle needs to be slightly smaller than the softball's diameter, so the size of the softball will determine the size needed for the circle.
Drill a small hole with a wood drill bit to make an entry point for the jigsaw, and cut the smaller circle out with the jigsaw.
Smooth the cut's rough edges with sandpaper. Paint the board if you want.
Place the board over the softball so that part of the ball is protruding through the top of the board. Mount the board one foot at a time, and you are ready to begin working on your balance.