You will need to have:
2 rubber bands
Step 1: Tying up
Take a rubber band and cut it, then tie each end to one pen
Step 2: Tie up
At the other side of the pens make the stay toghether by connecting them witha good rubber band.
Step 3: Shoot
To shoot pull back the cut rubber band while holding the bottom toghether. Use small pieces of paper or something soft.