First, pull off the writing part by grabbing the pen at the back, and with your teeth, grab the pen where it starts to taper. pull it out, and once its out, take the tapered part, and ink cartridge first, hit it on a desk until it comes out. now, you should be left with a writing piece, a grip thingamajig, and a blocked off tube. take the writing tip and insert it, ink cartridge first, into the blocked off tube. hit the writing tip on the desk by holding the tube. after a few hits, the end should pop off.
Step 2: Firing mechanism
take the balloon and put it over the tube, so that there is some loose balloon. take your tape and wrap it around the tube, taping the balloon to the tube.
Step 3: Arming
insert the writing tip, once again cartridge first, into the open end. grab the cartridge with the balloon and pull back. let go to fire(duh).