You only need a pen and a rubber band. It's that simple.
Step 1: Making the Pen Shooter
Take apart the pen. Then tie the rubber band around one end, as shown.
Step 2: Shooting the Pen Shooter
DO not point at people's eyes. Or people in general. This is powerful.
Load the inner part of the pen into the barrel so that it sticks out the end without the rubber band.
Fit the rubber band over the end of the projectile (this can be tricky).
Pull back and release and watch as the inner part of the pen flies in a long, straight line.