This is the instructions for an extremely simple spitball launcher. All it takes is a mechanical pencil. (The guitar pick in the pictures was just something so my camera would focus.)
Step 1: Disassembling the Pencil
Take the pencil apart. Simple.
Step 2: Making the Launcher
Take the internal piece and the tube. If you have to cut the end off to make it a perfect tube, do it. Then put the internal piece in the tube.
Step 3: How to Shoot
To shoot, put a spitball in the far end, and pull the internal piece about halfway out. Then flick it or push it forward.