I just dropped off my Form 1's at the Sheriff's office today for him to sign and return to me. Here was my process:
http://www.atf.gov/publications/firearms/nfa-handbook/
1. Purchase lower receiver
2. Send it to Orion Arms to have it engraved with your name/city/state
http://www.orion-arms.com/gun-engraving/short-barrel-rifle-engraving-locations.html (yes, despite what you may hear, you DO have to have it engraved).
3. Get two Form 1's from the ATF. Get two ATF fingerprint cards as well
http://www.atf.gov/forms/dcof/ Alternatively you can download the fillable .pdf here
http://www.atf.gov/forms/download/atf-f-5320-1.pdf and print it on your own printer. Make sure you print both pages on one sheet of paper (front/back). You can use two standard FBI Form 258 fingerprint cards supplied by your local PD. if you do this, you'll need to enter the BATF's ORI number, which is
WVATF1100
ATF-FFLC
Martinsburg, WV
4. Fill out Form 1's after engraved receiver is returned (debatable as to whether it needs to be engraved first, or get the NFA stamp first. Box 4h needs to match what is engraved on the rifle
5. Download and fill out one form 5330-20 Certificate of Compliance. Fillable .pdf is here
http://www.atf.gov/forms/download/atf-f-5330-20.pdf
5. Get your CLEO to sign the Form 1's
6. Attach a 2x2 passport photo to each Form 1
7. Mail to BATF with $200 check
8. Wait about 3-5 months
For an SBR, you are actually registering the lower receiver as the SBR. Technically you are the "maker" of the SBR even if Armalite originally made the lower receiver. Theoretically, you can't have a short-barrelled upper in the same residence as an unregistered lower as that constitutes "constructive intent" to make an SBR illegally.