Materials: China Marker pens, old credit card, rubbing alcohol, cotton ball
Wipe off all oil and dirt with alcohol, let dry.
I used the yellow marker for the 20 gauge (duh!), red for the 12 ga, and white for the 410.
Use the credit card to push the color inside the letters and scrape off the excess.
Dab with alcohol to "melt" the ink into the letters and seal them.
Took me less than 5 minutes to do all 3 barrels.