Warning: Do not use in a disorderly manner.
Step 1: Materials
1 You need the thickest pen you can find that has an opening big enough to fit a party popper in.
2 You need party poppers, I think I got them at Kroger or Walgreen's during 4th of July.
3 You need a knife or scissors.
Step 2: Take Apart the Pen
Next remove the spring and put the ink dispenser back in the pen.
After that, place a party popper in the pen.
If the party popper just BARELY doesn't fit, then roll the popper in between your fingers to make it skinnier.
If the party popper doesn't fit by a long shot, get a wider pen.
If the ink dispenser/holder is too long, cut some of it off with your knife or whatever.
Step 3: Finishing Your Exploding Pen
If you cannot, however, put the cap back on because the party popper(s) is/are holding it back, then cut more of the ink dispenser off.
A quick fix to that is to just use a little tape and tape the cap to the pen underneath the clip that holds the pen to a shirt pocket.