Next you want to get the knife from you pocket knife and start cutting the rim of plastic that was inside the body of the pen off. Be careful one knifes are sharp and two you do not want to cut the white plastic tube, thats where the ink is, if you cut it you'll have ink going every where. take you time and be careful.
you've done most of the work now we just have to put it back together. Its easy just slide the part you cut off off and put the tip back on and your done! Now when someone opens the pen, the top will either stick in the cap or fall out.