I make wooden blocks that fit around the profile of trigger guard on either (both) sides. I leave them longer than the casting, with flat spots so I can use a small clamp to hold them together. Then, I can position/ reposition the piece in a vise easily. I'll mortice out for the lugs to help hold the part as well.
I start with large bastard files progressing to small, finer files. I have one scraper called a "Pro-Prep" that really works well for finishing brass castings. It's especially effective for keeping facets crisp.
Butt plate is very tricky to hold. Again, I cut a wooden block to profile so I can hold it in the vise. If you have enough confidence to drill your mounting holes it really helps. I just drill them big enough for a drywall screw at this time. The scraper really shines up a butt plate.