First, download a couple of Form 1's here. [url]http://www.titleii.com/pdf/NewF1.pdf[/url]
Next get two Fingerprint cards from the ATF or your favorite C3 dealer.
Go see your CLEO, normally the Sheriff, and get his signature. Get printed while you are there.
Get two photographs and attach them to the Form 1's with tape.
Just in case I'd download this [url]http://www.titleii.com/pdf/5330-20.pdf[/url] and send it along with the Form 1's. It is a statement that you are a US Citizen. You probably don't need it, but its a little insurance the process won't be held up any longer than it already is.
Take all that stuff put it in the mail with a personal check for $200 then wait about 4-6 months. With a check you will get it back from the bank and know when its been cashed. Also, when the ATF says they never received it you will have proof on hand, not the hassle of trying to get proof of a MO from the USPS
Note: Unless you are using a preban receiver you still can't add an adjustible stock, flash supressor, etc. You can build an SBR, but not an Assault Weapon. Go figure.