Tape the Pencils
Start off by taping two pencils to each other so that they are exactly at a level. You should have two sets of pencils once you are done.
Make a T
Put the pencil sets over each other in a way that they make a T shape. Once this is done, tape them together firmly so that they hold well in place.
Remove any mechanism which the ball pen may be having, including the writing cartridge and the springs. All you will be left with is the outside tube. Do not throw away the catridge as it will be needed later on.
Tape the Tube
Now tape the tube with the vertical side of the crossbow. Place it above the pencils. Make sure that it is firmly in place.
Add the Rubber Band
Place them on the vertical arm of the of the crossbow in such a manner that it fits between the gap of the two pencils. It should be firmly held in place.
Tape the Rubber Band Sides
Use the paper tape to tape the two sides of the rubber band going over the meeting point of the set of pencils. It should be an extended taping so that the firing mechanism is easy to use and does not hurt the fingers.
Load the Cartridge and fire
Now load the cartridge into the tube. Once the cartridge is in place, pull the rubber band back behind the cartridge and fire it with full force. Once done, you can put the caps on the both ends to store it for later use.