Instructions
Fire a test shot at your target. Catapults almost never hit the target on the first try, so you will need to bracket the target. You will adjust based on whether the projectile lands long or falls short, or whether it drifts to one side or the other due to wind or other factors.
Release the projectile early for a higher, shorter trajectory. Release it later for a flatter, longer trajectory.
Adjust your projectile's weight. Heavier objects loft higher and fly shorter distances. Lighter objects fly flatter, faster trajectories.
Observe carefully where each projectile lands. Continue to adjust your release time or projectile weight until you have zeroed in upon your target.