It is very simple to make. First you need to take a pen apart. The only parts you need from it are the tube and the ink cartridge. Cut the tube in half or whatever size you desire. Cut the rubber band so instead of a loop, it is one long strand cut the rubber band down to size so it fits around the tube and there is a little room near one opening of the tube. Tape the rubber band to the tube. You might need to use several layers of tape to make it stay. Make sure the tape is on tightly. To shoot the gun, you pull back on the ink cartridge and the rubber band and let go! Don't shoot at humans, animals, glass, or anything you don't want to break. I recommend shooting into a card board box.