[edit] Steps

1. 
   Line up two of the pencils parallel to each other and secure them with tape, leaving at least a centimeter untaped at both ends in order to leave a space for the rubber band later. Repeat for the other two pencils.
2. 
   Create a cross that looks like a lowercase "t" with the two pairs of pencils. Secure them in that position with either tape (electric tape works best), rubber bands, or glue.
3. 
   Unscrew the tip of a cheap or useless pen. Unscrew the other side of the pen, and then take out all of the inner mechanisms. (Save the ink cartridge because that will be used as the ammo later.) After doing that, you should end up with a hollow tube.
4. 
   Secure the tube to the vertical side of the cross with tape.
5. 
   Stretch a rubber band horizontally across the crossbow, anchoring it between the two pencils and going over the pen in the middle.
6. 
   Wrap a piece of tape around the middle of the rubber band where it passes over the pen.
7. 
   
   Tie a rubber band between the pencils and the bottom of the hollow pen. This is to add elevation, which will come in handy later.
8. Put the ink cartridge back into the tube so that it's sticking out slightly.
9. 
   Stretch the rubber-band to the end of the tube.
10. 
    Grab the ink cartridge, pull it back further with the rubber band, and let go.

[edit] Tips

• Try experimenting with different types of bolts (the "arrows" used by your crossbow). Some bolts might work better than others, due to their size, shape or weight.
• The bow isn't very good at long distance shooting, so if you want to pierce something, shoot it from very close up.
• Use a bendable ruler for the base. It will give a little more flexibility, offering more power when the projectile is released.
• Use a binder clip as a trigger.

[edit] Warnings

• Do not aim this at people or animals, even as a joke. Accidentally letting go of the rubber band from something as simple as someone bumping into you might cause a serious injury.
• If made poorly, the crossbow won't work well and will be hard to handle.
• Do not look into the end of the crossbow while loaded. Use common sense.