Were do i start the process to get a sbr lower registered
Well, BEFORE you possess one, you apply to the BATFE either to transfer one to you, or apply to make one. Submit the paperwork, prints, OK from your local chief law enforcement officer, and a $200 check. WHEN it is approved you will get your papers from the BATFE with a pretty blue $200 stamp attached.
THEN you can have one. If you already HAVE an SBR, and have not touched second base with the BATFE, you have committed a Federal Felony good for 10 years and a $10K fine.
That includes possessing a short upper and a stocked lower, unless you have a pistol lower to go with the upper. Called constructive possession, even if they are not assembled. Tread VERY lightly here.