I rough finish with a 6"mill file, generously rounding all "outside bend areas of both the plate and STA to avoid accelerated wear on the wing and drysuit. (The side of a bend with a single press contact line is the punch side or inside, the side with 2 contact lines is the outside or anvil side.) The corners" of the plate, or the large radius at the top and smaller radius at the bottom, should also be well rounded. I do a generous break of the "handhole" areas as well to provide a nearly smooth radius from inside to outside. All other "outside edge" surfaces get an approximate .03 x 45° bevel on both surfaces of the plate with a file. This is about 4 passes with the 6" mill file the way I do it. I then break the edges of the slots to the extent practical with the file. There is not much you can do here with a larger than 6" file on the heavier plates due to the plate edge on the other side getting in the way. For those a Dremel a flame grinding bob will help, but its not absolutely necessary. It's about a wash to use the Dremel time wise, but easier elbow grease wise. A countersink makes quick work of deburring the holes, or you can use the flame bob. Most of the edge holes don't need a significant deburr unless you intend to use them as lashing points to attach bolt snaps.
All edges and slot internals are then smoothed and blended with 80 or 100 grit cloth back abrasive strip used "shoeshine" fashion until the edges are blended. The strip should enter and exit the slot as close to inline with the plate surface as possible. I use the "worn out" strip to do the final polish, as it wears down to about 400 grit. All you need to do to the slots is to break the edge a bit. Remember the strap isn't supposed to be moving in the slot once the plate is set up, and a "semi-sharp" edge to the slot is self-locking. They should not "feel" sharper to the thumb than a butter knife blade though. Be sure to thoroughly blend the outside edges and plate corners.
A final wash with dish detergent and a Scotchbrite pad removes any surface oils and abrasive residue.
When holding the plate in a vise (I _DO_ suggest using a vise or other method to clamp the parts to a workbench while finishing!) use an old leather glove or piece of shoe leather to cushion the plate in the vise jaws. If c-clamping to a table for deburring use a towel or leather glove on both sides of the plate to avoid damage to the plate or table.
I do enough of them that I use both 2" wide and 1" wide Norton shop roll aluminum oxide abrasive strips, a Severance chatterless countersink, 4 different size files including 6" and 10" round and flat single cut mill files, a 3M Rolock craytex finishing "brush" in coarse grit, and the same coated kevlar Perfect Fit II gloves I use when diving. It can be done with just a 6" flat file (you need one that small to get into the slots) and about 4 sheets of 100-grit emery cloth cut into strips lengthwise. If I remember correctly Harbor Freight usually has 1" wide x 50 yard imported shop rolls for under $10 each. They don't last anywhere near as well as the Norton or 3M stuff but you'll still have enough to finish a couple plates and STAs on a single roll with material left over, and they cost 1/3 as much as the Norton.
