Step 1: Parts needed
Eight cheep white rubber erasers--$1
One roll duct tape--$1
Plastic utensils --you probably have them already, one knife, one spoon
The erasers are just for absorbing the impact of the mouse trap bar so that the catapult will throw projectiles in the right direction. There are probably many options for substitutions.
Any normal mousetrap should be adequate, the same goes for the plasticware.
Use only the finest duct tape.