You will need 2 passport photos. You will need two of the the blue ATF fingerprint cards. The signature of your Sheriff or Police Chief or applicable CLEO on your form 1. You need that certificate of citizenship.
Important! when you print off the Form 1, make sure you print it front and back. The ATF can and will reject it if it is on two pages.
Very important, don't shorten the barrel untill you have the approved form 1 in hand and you have engraved the gun. The receiver is now your SBS even if you put a 26 inch barrel on it, it is still a SBS. you may not put the short barrel on another gun even if you put your engraving on the barrel. the receiver is the NFA item.