Not easy, but simple instructions:
Find what you want to buy and make a deal with the seller. Bring money. Lots of money.
Dealer will help you with the paperwork. You'll get two identical forms with address info, serial number(s) and description of the item you want to buy, dealer's information, etc.
In the packet you'll find a set of two fingerprint cards; take these to a law enforcement agency and get two sets of prints done, making sure that the agency name and officer's name are in the proper box. Get two wallet size photos to glue on the forms, one on each.
Take the completed forms to the chief law enforcement officer of your county (or city) and have him/her sign them. This is tricky, since some LEOs won't sign them.
Sign and date the forms and mail them to ATF along with a check or money order for $200 per item. (If you buy a suppressor to go with your gun at the same time, for instance, you'll have two sets of forms, four fingerprint cards, and four photos, along with four signatures from the top cop. And a check or money order for $400.)
Wait. And wait some more. Time is variable from as little as six weeks to even as long as six months. At some point the dealer will receive your tax stamp(s) and give you a call, at which point you can go pick up your new toy.
Go to Wal Mart, Academy, Vann's or wherever and buy lots and lots of ammo.
Waiting on a stamp for this one and another very much like it. MK760 9mm SMG.