Once side profile is "close", I measure length of pull and cast-off, then install buttplate while buttstock is still square (and true). I use a coping saw, flat and round rasps for buttplate. After buttplate, I hog off wood using the rough side of a horseshoe file. After that, it's finer rasps, flat and round, and then sanding block, then finally hand-held sandpaper. Constantly checking with a straightedge or some other kind of measurement.
Of course, each step is done while entire gun is in the same stage of finish. Bill