You get permission from BATFE to make an SBR. Once you get that (Form 1) you can make the gun; it was already registered when the Form 1 was approved. Note that you, and only you, can make the SBR. If you use a gunsmith, you don't need the Form 1, but the gunsmith must be licensed as an NFA manufacturer and you must take/send the gun to him; he will alter it and return it on a Form 4 after BATFE approval of the transfer.
Jim