Caveats: I'm not a lawyer, I don't know dick about MA-specific law, and I know even less about SBR-ing AK's.
Briefly (As in, many details and subtleties are left out, so it's...
brief):
1. Establish a trust. There are many ways, including via online options (199trust.com, GunTrustDepot.com, Wildcat Trusts, etc.) or a breathing lawyer (RockMaple here is one such who comes highly recommended). Think very carefully about your trust name. You will have to engrave your lower with the complete trust name. "XYZ Trust, Taxme MA" is a subtle engraving. "The John D. and Catherine T. MacArthur Revocable Living Trust, Taxme MA" is less so. Your name isn't required; it could be initials, or anything you want. If I had to do mine over, I might choose "This We" or "Power We Can" or "In God We" so my lowers would be engraved "This We Trust," "Power We Can Trust," or "In God We Trust."
2. Get trust notarized. Most banks will do this for free if you're a customer. Otherwise, it's small bucks. If you're using a breathing lawyer, he'll probably handle this with you.
3. Fund the trust. That means assign something to it. Some people use $1. Some people use the lower they're going to SBR. Anyway, a trust must be funded for it to be valid. In most trusts, you do this via an Assignment Page. Again, if you're using a breathing lawyer, he'll probably handle this with you.
4. Scan your trust documents: the Declaration of Trust and Assignment Page. Make sure the file size doesn't exceed 3mb (IIRC), so you might have to play around with scanner settings, or some other way of making the PDF smaller. PM me if you want my technique. Too geek to describe here.
5. Obtain the lower you're going to SBR. You'll need the make/model/serial number for the Form 1. General rule of thumb for an SBR AR15 for OAL is 19" plus barrel length.
6. Open an account on BATFE eFile. (
https://www.atfonline.gov/EForms/) Only don't do it today, because eFile is down on Wednesdays from 0600-1800.
7. eFile your Form 1. There are numerous "How-to" walk-throughs on the 'Net, including YouTube for the reading challenged. It's not hard, and basically plug and "pay." You pay your $200 transfer fee right there online when you file.
8. Have your lower engraved with your trust name and city, state. There are a bunch of threads on NES as to where to do that locally.
9. Wait. A lot. Currently around 100+ days, and as 13 Jul gets closer, my guess is...wait longer.
10. Some day, an email will drop into your inbox from the BATFE with a "status update," with an attachment (that won't display on your i-
anything) that is your approved Form 1.
11. Build SBR.
You said brief, and the above is just the bare bones. If you've got any questions, let fly. I'm not an expert, but I've got
a number of SBRs under my belt at this point, all via trust, and all but one eFiled.
ETA: Update on 4: I think in MA you have to submit your LTC or something, too. As I said, I'm not up on MA-law. Check with other MA-smart people on that one.