Follow the below link,
Enlarging Sizing Dies...
I used some wet dry and added in a touch of 3-1 machine oil. Just rolled it back and forth on my leg like mentioned. It took about half an hour or so to open it up .001" using some 280'ish grit, the another half hour to polish it up really smooth and shiny with some either 6 or 800 grit. I was watching something stupid on the tube while I did it, so it didn't seem too long. I only needed it opened up a touch in the first place though.
Just go slow like they mentioned and every 15 minutes or so, (or between long commercials) run a rag through it and then size a bullet to check progress. You don't have to put much pressure on it let the grit do the work and less pressure keeps it from scratching too deep as well. Just remember, it comes out easier than it goes back in....