First you need to find out what class III items are legal in the state where you live. Once that is done find a class III dealer, they will be of great help to you.
Choose your toy and pay for it and the $200 tax. You will be given paperwork to sign and then must get passport photo's made and be printed by your local law enforcement. They get copies of the paperwork and do a background search and if all is well sign saying it's good. You then send copies of that to the BATF and wait.
It's preety simple actually.
The hardest part is coming up with the money and making sure you find someone in the local law enforcement who will approve you, once that is done (assuming you are legal and have no criminal history) you just bide your time.