Fill out ATF form 1, send it in (with your check for $200) to the BATF and wait about 6 months for the stamp to arrive in the mail with your approved form 1. Until then, remove the barrel and keep it someplace until the paperwork is returned by the BATF. I did a 9 1/2" SBR build last year and that's how long it took my stamp to arrive. I don't know where you live but normally you don't need a FFL holder to do anything for you if you already legally own the gun. Just get the necessary ATF forms, complete the requirements, send in your check and wait for the feds to to their job.
"... Americans... we want a safe home, to keep the money we make and shoot bad guys." -- Denny Crane