Do you you have to a have a Class 2 FFL put it together? Could one just have a short barreled upper and lower but not assembled? What is the proper procedure?

So, I just file the form 1 to register a lower receiver as an SBR? I can then put any upper on it after getting the stamp?

You just need an upper receiver block and a barrel wrench.

Well almost.  If the gun was a post ban before registering it as an SBR, its still going to be a post ban after registration and can't have a bayo lug or flashhider (nor can you attach a telestock).  I have heard it rumored that BATF is now taking the position that registration as an SBR is making a "new" firearm, and thus all SBR's registered after 9-13-94 have to be in a post ban configuration.