Dude, Step 1, you really need to locate a Class III SOT in your area. Post on silencertalk.com and ask for a dealer in your city. You'll find one there. The dealer will likely know how to interact with your local sheriff and will provide you with the Form 4. There may also be a state registration process that your dealer will know how to handle (or there may not be).
Step 2, you need to turn 21.
Don't get into a big pissing match with the Sheriff yet. You are pulling the trigger and panicking too early on the signoff. Buy the suppressor first from a dealer. If its not in stock sitting on his shelf you have MONTHS to wait until you have a Form 4 ready to sign.