Dude, this is a PITA. You have to go to downtown Phoenix (Jefferson) to the county courthouse and submit the paperwork. Actually, in addition to the ATF forms/fingerprint cards, you have to complete AND HAVE NOTARIZED two additional special forms just for Sheriff Joe, plus an extra photo, plus an additional fingerprint card (i.e. 3 in total). Make sure you call ahead and find out the best time to get NFA fingerprints done, or you will be in for a long wait (when I did it back in February, they recommended coming Mon/Tues/Thurs between 9am and 2pm). Oh, and don't attach the photos to the ATF forms, as they won't go thru Sheriff Joe's printer. Having said all the above, the people actually handling this for Sheriff Joe are really helpful and efficient.
By the way, if you are having a 'smith make the rifle for you, do your homework on whether you should be using a Form 1. If you truly are the maker (i.e. you are ordering a new upper to attach to your existing lower) then a Form 1 is appropriate. If, on the other hand, the local 'smith is doing the whole build and delivering a completed rifle to you, then I think you need to complete a Form 4 and the 'smith needs to be a SOT/Class 3 manufacturer.