Get two ATF Form 1s, for "making a firearm". Fill 'em out, get your chief law enforcement officer to sign off on 'em, get two sets of fingerprints taken on official ATF fingerprint cards (they don't like regular police cards, they want their own) and two passport-size pictures. Also fill out a copy of the "yes I'm a citizen" form. Send all that crap, along with 200 dollars, to ATF. Sometime between 3 weeks (the shortest I've ever heard of) and 13 months (that's how long my first one took), ATF will (hopefully) send you back one of the copies, with a stamp on it. They keep the other copy. Once you have the stamped paperwork back, you can then (and only then) buy that shoulder stock and put it on the pistol.
If you have a stock, and you have a pistol, even if they two aren't together, ATF has a tendency to consider that a done deal, and if you don't have your paper yet, that makes it an "unregistered NFA item", and they arrest your butt.
The address to send the check to is on the form.
Meddle not in the affairs of dragons, for thou art crunchy, and taste good with catsup - George of Lod, Year of Our Lord 297
I always take precautions.
Beware the Evil Bullet Fairies.