At this point, I added the neck gussets and pinned down the shoulder straps, then ran a loose running stitch through the neckline, in order to gather the neck into the collar.
The back of the collar was pinned down the rear of the shirt and sewn down:
raw edges shoved into the collar opening (including the shoulder strap), then collar applied with a whip stitch on the shirt interior. The whole shebang was then top-stitched.
