Start with a Tul pen that you can get on amazon here: http://www.amazon.com/Retractable-Needle-Point-Black-4-PACK/dp/B004IG53QS/ref=sr_1_11?ie=UTF8&qid=1296011064&sr=8-11 or just google it (it should be pretty easy to find it).
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Step 1: The Look of It
This is how your pen will turn out in the end.
Step 2: The Look of It
This is what it looks like with the cap.
Step 3: The Parts
You should get these parts when you break the pen apart.
Step 4: 2 Lego Pieces Added
Plus 2 lego pieces you will need.
Step 5: Glue Needed
And any old super glue.
Step 6: Step 1 Glue
Apply glue to the inside of the cylindrical lego piece.
Step 7: Step 2 Insert
Insert the pef half way inside. Let it dry.
Step 8: Step 3 Cut
cut the ink cartridge (see picture above to see measurements).
Step 9: Step 4 Add
Put the ink cartridge inside the lego peg.
Step 10: Step 5 Finish
Insert the part you just finished into the front of the pen
Step 11: Step 6 Final Product
Now add the cap back on and your done!