http://www.youtube.com/watch?v=ZQfm0nLXJ7Q
This is part one of three (kind of boring) videos on grain filling. But what is important with this series is that he will probably answer a lot of your questions, and most importantly, you get to see how how the pros do it.
Don't forget, you can save the old grain filler-when you scrape it off the cabinet, toss the excess back into the can.