That you are not a speed builder is a very good thing. I know you are anxious to see the end result, but it is imperative that you take your time and do each process right and well.
Start by clamping the guard in some sort of fixture to hold the guard. A temporary one can be made out of 3/8" fir plywood. Cut the top edge at the same angles as the extensions of the guard, and cut out the part in the rail and bow so that the fixture clamps only onto the attaching lugs.
Now file all surfaces with small fine files to remove parting lines and the cast finish. Next, backing abrasive cloth or paper with either wooden pieces or the same files you used, remove all the file marks. Do this step with abrasive cloth or paper in the 80 - 120 grit range.
Now remove those marks left by the first abrasive with 180 grit, stroking at an angle different than the first, so you will leave parallel scratches of 180 grit and remove all those left by the 120.
Now stroking at another different angle that may be the same one you used with 120, remove all the 180 grit scratches with 220 grit abrasive.
Continue this process going to finer and finer grades of abrasive until you end at at least 400 grit. Now burnishing with 0000 steel weool will give you a nice finish. If it still says to you it needs a finer finish, go as high as you want, ie: 1200, 1500, or 1800 grit. I think that's overkill, though I often finish at 1200 before burnishing.
One thing you'll find...each successive grit will go faster and faster, removing the marks left by the previous abrasive. But it is imperative that you remove all the marks left by the previous paper/cloth before moving along.
Do not be tempted to use a buffing devise. Nothing ruins hardware more quickly.
The butt plate can be mounted on a scrap of wood cut to roughly fit the inside curves, and screwed down gently. Thimbles can be held on a drill shank that will just pass through the hole, or a tapered holder like a rat tailed file. Nose piece gets polished on the wood. Side plate likewise can be screwed down to a scrap of wood so it can be held flat in your vise.
Now the lock...start with the plate. Here, stones might work better for you. I don't have any, so use my system described above for the lock and its parts.
Here's a trigger guard I made for my Wm. Antes swivel project, using a sand cast guard I bought from TOW, There's lots of brass in those sand cast guards!!