If the gun is in state, have the seller fill out 2 Form 4s with his proper info etc. Then you will receive those Form 4s get your proper signatures (if needed), fill your info, etc. Mail it off with your $200 check, (fingerprint cards, passport photos attached; once again if needed) & Certificate of Compliance. Wait for the transfer usually 3 months. The paperwok comes back to the seller at which time you can pick up the gun.
If the gun is out of state, you will need to find an in state FFL who is licensed to do NFA items to do the transfer for you. Same process as above, just with a dealer.
If you go the individual route you will need to get your Chief Law Enforcement Official to sign your Form 4s, get passport photos, and fingerprint cards done. If you're gonna buy through a trust or a corp, you dont need the CLEO signature, photos, or fingerprint cards.