If you want some tutorial information on how to use them, the Stew Mac site, has some good videos as does Youtube. Stew mac also sell some really good specialist guitar scrapers but they are quite expensive, very good tools though.
All you really have to remember is to go with the lay of the grain, not against it and make sure that you keep them sharp.
If you are going to go minimalist and use the Knife blade, wear gloves, and keep it more or less square to the surface you are scraping. You are best to try on a piece of scrap (one of your headstock off cuts) until you get the feel for it.