If you're building the SBR as an individual:
You fill out an ATF Form 5320.1 in duplicate. Each one requires an in INK signature. So if you're photocopying it, sign it AFTER you copy it. While you're at it, fill out a 5330.20 as well. Commonly called a "Form 1" it's the form that you need to fill out to build a SBR.
You will need to engrave the receiver with your Name, City and State. You need to get the fingerprint cards, and the photos for the form 1 and take it to your local Sheriff or Chief Law Enforcement Officer to have it signed off.
Then you send the Form 5320.1 in duplicate with your passport photos, finger print cards, and a 5330.20 to the ATF. Then you go ahead and wait. A couple months or so later, you'll get your approved 5320.1 back in the mail and you can go ahead and buy your stock.
If you're doing it as a trust or LLC:
I presume you already have the Trust or LLC set up with the appropriate paperwork.
Fill out the 5320.1 in duplicate. Forget the finger print cards, or the photos. Forget the CLEO signature as well. (Trusts or LLC's don't have fingers or pictures.)
You can then engrave your Receiver with "XXXX, Trust" or "XXXX, LLC."
Then you send your Form 5320.1 in duplicate to the ATF. Some examiners STILL require the 5330.20 but some of them don't. You may as well send one in anyway. You need to send a copy of your trust with the Schedule A, or a copy of the certification of the LLC.
Once you have your sbr registered, you can go ahead and add a fore grip any time you want to.
Hope this helps.
RJ