Step 1: Dismantle the Pen
first you must get an average BIC pen and a rubber band. we will use the rubber band later. you can throw away the cap and the back part of the pen. by the bottom part i mean the little cap that covers the back of the pen. this pen barrel is hollow. for the back you might need to use pliers (preferably needle-nose) to take it out.
then take out the ink tube. just bite the little brownish part on the tip of the pen and pull if you don't have long nails. leave the little black thing at the top. if you don't it will be a bit harder to shoot.
now stick the bottom of the ink tube into the end of the barrel without the black thing like in picture 3.
it should now look like picture 4.