To get the popper string through the hole in the back of the pen, straighten the paperclip completely and attach the string to the end either by taping it on or making a hook at the end of the paper clip. Once you have the string attached feed the paperclip through the pen and use it to carefully pull the string out the other side. Don't detach the string and paperclip yet, as the paperclip can make the next part easier as well.

Now that the string has been pulled through the pen, pull the string through the cap as well, then push the cap onto the pen and pull most of the slack out of the string. After that, detach the string from the paperclip and use the clip to push the free end of the string back into the cap through a different hole. If done right, the poppers should be seated in the back of the pen with their string coming out through the hole and looping around something in the end of the pen cap before finally ending within it.

To secure the string in the cap, apply a pea size amount of hot glue through the cap's vents. If you don't have a glue gun handy, you should seriously go out and get one, but I suspect epoxy or even elmers glue would do the job. The outside of the cap will be visible of course, so if you want the finished gadget to look good take care to keep the glue job neat, you may even want to remove the glue from the outside of the cap with a knife after it cools.

Now that the firing mechanism's done, use the bit of paper towel to make 2 or 3 wads about 30% bigger than the barrel of the pen, then push them as far back as they'll go.

Using the paperclip or something similar, measure the remaining depth in the pen barrel, then cut down the ink tube to fit.

Be careful if you have to cut past where the ink starts, as some of it will likely escape (I learned this the hard way, in case you haven't noticed my hands).

Dabbing the cut end on a paper towel a few times will help, since it'll pull off some of the ink that surface tension won't hold.

Once the writing assembly is cut down, use a blade to shave a bit off of the fitted surface of the writing head, trim it down little by little until it can be removed from the pen barrel easily, but not so easily that it's prone to falling out on its own.