If you've read my paper dart tutorial, this and the following steps will be pretty familiar.

First, attach the edge of a piece of scotch tape half a strip's width longer than you'd like your fins to be along the ink tube as far back as possible. I don't know what the optimal length for stabilizing fins is, but too long is better than too short, so I err on the side of long.

Next, fold the tape over, so as to stick its other edge along ink tube.

If you're going to make 3 fins try to use less than 2/3's of the tubes circumference for each fin, any more and they won't fit cleanly, adjust accordingly for more fins.

Repeat the above process for each fin, then carefully trim the fins into triangles, leaving half a tape width or so with no fin at all at the forward end.

Finally, take a length of tape and wrap it around the finless portion of the fin tape, Start slightly high and wrap a couple of mm's rearwards so as to keep the tape from unravelling at high speeds.

The addition of this roll of tape may not seem too significant, but without it, your fins will usually be ripped off by the airflow around the dart within a few feet of leaving the pen barrel.

And that's it! Put the writing assembly back in the pen and you've got yourself a nifty concealed dart gun. In case you haven't already guessed, to fire it just point it where you want it to shoot and yank the cap off the back. If all goes well, it should make a nice bang, and launch the pens now aerodynamic head and ink tube a good distance.