You don't have to have a trust- you can file as an individual (but trusts do have their advantages). These days, along with the Form 1, you're going to need to have fingerprint cards done by your local law enforcement (The Probate Court here sends you to the LEC (jail) to get the cards done). I think it's a $30 fee.
And you have to have photos- passport type pictures. USPS or Walgreens/CVS can do those I believe.
And all that gets sent off with a $200 check for the actual stamp. Then you wait for months and months until the stamp comes back... then you pay to get your lower receiver (if it's an AR pattern rifle) engraved with the trust name and city/state. There are several places that will do engraving (ship it to them and wait) in the $80 to $110 price range (for engraving alone).
Then, with the engraved lower in hand and the tax stamp in hand, you can assemble your SBR upper and newly taxed/engraved lower.