I've heard lots of methods, but there's only a couple that work for me:
1. I use a piece of white chalk pastel that has been sharpened in a pencil sharpener. You can pick out the lettering quite easily, and any mistakes can be wiped away with a brush dipped in alcohol). Blow the dust off and then spray with flat-coat overtop. This method gives me the best results.
2. I've also had moderate success by putting some white paint on an index card or a post-it note. I then 'dab' the raised lettering in the white paint. It's a bit of a hit-and-miss technique, but when it works, it works fairly well. The biggest challenge is getting the right amount of paint onto the card. Too much will just make a mess and too little won't leave a good impression.
3. If you're trying to paint the letters in with a brush, I'd suggest a couple of things:
a. Use a magnifying glass of some sort. You will have much more control.
b. Use a brush that has long, thin hairs (I think they're called 'liner' brushes?). They hold a good amount of paint and really work well for this type of work.
Good luck.
M.