First, make up a little wooden jig out of a piece of 2x4, with a cut out the shape of the buttplate mortice. Attach the buttplate to the wooden jig so that you can easily manipulate it in the vice. Start with rasps (if lots of material removal is required), then mill file, etc. I drawfile the parts when possible, which gives a very smooth finish. I can go from drawfiling to 220 grit emery paper, then 320 etc. This goes pretty fast with brass, somewhat slower with steel.