If you create a regulated item (firearm receiver, short barreled rifle, "silencer") from raw materials with no existing serial number, you must submit a Form 1 to ATF describing your business, describing the part, identifying the manufacturer, model and serial number you have assigned. You can't manufacture it until the ATF approves the Form 1 and accepts the $200 tax. If you built using an already manufactured item e.g. an Anderson AR-15 stripped lower where the manufacturer, model and serial number are present, then you just enter that information on the Form 1. Again, you can't assemble the finished part until you have the approved ATF tax stamp in hand. If disapproved, the Anderson lower can still be assembled into a non-NFA regulated firearm e.g. a rifle with a 16 inch barrel on the upper.
If your Form 1 creation is part of an NFA trust, you need to engrave the trust identification on the part with engraving deep enough to meet ATF requirements. I paid a local FFL with a specialized CNC milling machine to engrave my lower (still stripped to ease placement in the CNC mill).