everyone here seems to do it differently lol what I would do (if the hair is virgin) is foil back to back, backcomb about 5-6 inches down (or wherever you want the colour to start) then put the darker blonde for a few inches then blend a lighter blonde for a few inches etc til your lightest colour is on the ends. Depends on how dramatic you want it to be really!
have a search on here for "ombre" technique, you'll find loads of info x