Here's what you're going to need:
-Wood Scraps (I used less than 3 feet of wood that were scraps from other projects)
-Wooden Dowel that fits into the PVC
-Halloween Mask (try one that's light weight)
-assorted screws and a drill.
Heres the environment that this was used in to get you familiar with what I was making here. I would have two witches standing over a fogging cauldron (garbage can). Trick or treaters would be brought up to the scene by a guide. The witches would start chanting, and at the right moment, the guide would set the catapult off, sending the skeleton mask flying out of the cauldron towards the trick or treaters...
Below, there is a sequence of pictures of the mask popping out of the cauldron.
It scared several people this past Halloween...
Step 1: Make the base of the catapult...
Here I just took a the smaller piece of wood (1x2") and screwed it into the larger piece of wood. I used two screws on each end of the wood strips. Be sure to drill pilot holes so the wood doesn't split.