i used a medium tip Paint Pen from Elmers. Trust me they work perfect. I actual wiped with Acetone on a paper towel similar to the video and it worked without taking away the paint from the letters.
Wal Mart sells the Elmers paint pens in 5 packs... can't get much easier than that.
@ Tim D, I used a mini sledge. And Trust me you want to hit it pretty hard. If you don't it's hard to get the stamp in the right place the second time and it will look sloppy.