Many times I am asked about how a trust (or the settlor) can change an AR-15 rifle into a legal Short-Barreled Rifle (SBR). The process is easy, albeit long and exact.
If your Trust wishes to manufacture a NFA firearm rather than purchase one, it will need to file a Form 1. The Form 1 must be filed, approved, and returned before manufacturing (i.e. converting) the firearm. The fee for the Form 1 filing is $200.
The Form 1, which can be found on the BATFE webiste, must be submitted with a copy of the trust and assignment sheet, a certification of US citizenship for the submitting trustee, and a check to cover the transfer tax. Unfortunately, you should expect to wait nine or so months currently to have the form approved.
For Form 4 transfers I advise trustees to leave the item off of the trust assignment sheet until the BATFE approves the transfer. However for Form 1 manufacturing the title 1 firearm (or block of steel / aluminum if you are really ambitious!) can, and should, be assigned to the trust before manufacturing begins. (Arguably this could be at any time, but I suggest assigning the firearm before submitting the Form 1).
Once you have received the approved Form 1 and begin manufacturing the firearm you must engrave the firearm with the caliber or gauge, make, model, name of the manufacturer, i.e. the Trust, and city, state in which the manufacturer is located. You should use the full name of the trust rather than abbreviations. The BATFE currently requires a minimum depth of .003 inches for the engraving, but I caution anyone manufacturing to verify the current requirements to manufacture before pursuing a Form 1. Due to the exactness of engraving requirements, I highly recommend taking your firearm to a Class III licensed manufacturer to have the engraving completed.
It is worth noting that the manufacture of machineguns for civilian use has been illegal since May 19, 1986, and the BATFE will not approve any request to manufacture a machinegun. (In the event the ATF does mistakenly approve such a form, they will confiscate the firearm once they realize what has happened.)
As always, if you find yourself confused or need of guidance, reach out to someone with experience. It will make the process much simpler!