I use a bench grinder. A dremel would work too.

All you need to do is remove the thickness of the rim plus the amount the bearing protrudes above the kit surface.

BTW. Don't spend $8 on a kit. I make mine from a 357 or 38 special case that I pick up at the range. cut the sides of the case down to the case head. I remove the primer and glue in a $0.16 ball bearing from the hardware store. IIRC it's 5/32".