It's actually very difficult.
1) get a white (or whatever color you like) Crayola Crayon.
2) Rub it over the lettering.
3) Wipe off the excess.
4) Repeat as you feel like it.
It actually does a decent job. It stays on pretty good. This example has been exposed to Rem Oil and it still looks good. But it won't work for parts that get hot. Warm, yes, hot no. In fact, it comes off more when its cold than when its warm. It's easy to clean off if you get tired of it. A little hot water and it comes right off.