I've done this to a lot of my guns. Pretty simple:
You will need a lighter or some source of heat, crayons of your choice, some paper or business cards, a couple patches (the gun cleaning kind), and some remoil or similar.
Slightly heat the area you want to color. Remember you're melting wax, not steel, don't go overboard. Start coloring the letters..hard. Fill the depression entirely with wax. Sometimes it helps to do some, let it cool a little, do some more; you'll get a feel for it. You can also heat the edge of the crayon a little or melt the big blob of wax you make into a puddle that will run into the letters.
Go in all directions and use good light to make sure you fill it up. Grab your business card and run the flat edge along the are you just colored in a scraping motion, taking off some of the excess.
Now let it cool for a while. Use the business card again and rub the area colored briskly. The heat generated will melt the excess wax and transfer it to the business card.
Do a final exam with a good light to make sure you got everything. Put a little oil on the patch and wrap it around your finger. LIGHTLY rub the colored area until the last of the extra wax residue comes off.
Voila!!