Think of an object or character you want your castanet to look like. Go with a shape that fills as much of the surface of your folded cardboard—perhaps a flower, a seashell, a cat's head, or the face of a favorite cartoon character. Draw an outline on the front panel of the folded cardboard.
If you're using a paper plate or if your cardboard castanet is a lot larger than your palm, it is best to attach a pair of straps for an easy grip.
While keeping the paper plate or cardboard folded, punch 2 pairs of holes an inch from the folded edge. The distance between the holes should roughly be the same as the width from your little finger to your pointer finger.
If you did Step 5, attach straps by cutting a pair elastic bands or ribbons that are about 2 to 3 inches longer than the distance between the holes on the paper plate. The idea is for the straps to fit snugly around your four fingers.
Take one strap and insert the ends through the pair of holes on one side of the castanet.