Making a Catapult

So yesterday, We bought two side supports for the catapult (15 inches long, pine wood)  and then we bought a longer piece of pine for the swinging arm and it was 2 feet in length. We also bought metal triangular pieces (pictures later) to make the sides of the catapult.  We also bought hooks to attach a rubber band to this swinging arm. Yesterday, we marked holes on these three pieces of wood one inch a part, in all but the base piece.