I haven't built a car model in lots of years, but I always just used a toothpick on the letter. Spread a little paint in a cap of some sort (all you want is a very thin layer), stick the end of a toothpick in it, and touch it to the letter.
You can also dry brush them. Use a rather stiff brush, put a little paint on it, and then wipe as much of it off as you can. Carefully paint the letters and it should only highlight the tops of them. It may take a few coats to get good coverage.