You will want to assemble it first starting with the barrel then the rest of parts to check for fit and function. Take notes before disassembly where and how much wood needs to be removed to slim it down like sides of forearm at muzzle behind cap and grip area to fit your hand and such. Then take it apart and file and sand as needed, raise grain with damp cloth and wait till dry , sand off whiskers with 320-400 paper . Alchohol stains are best in my opinion as they dont raise the grain again. Tru Oil is a good all around finish and is easy to apply .
This is the quick version , I think there is a tutorial in gunbuilding forum .