It's not to dificult, what i do is clamp the ring to a project table i have, theres a cut for a file in the table, i clamp the ring aligned with the cut and file like that it files even every time!
It's pretty difficult to do correctly without the filer. Keeping the ends square is just too hard to do with a hand held file. I agree with SG, buy it once and you'll have it forever.