How do you make a ar 15 fully automatic?

There's two ways to start this process. The first is to obtain a Class III NFA tax stamp with the purchase of a registered Class III lower receiver or trigger group. The second is to obtain either a Type 7 or Type 10 FFL with Class II Special Occupational Tax Stamp. Those are the first steps to go about it the legal way. Guides on how to go about it in an illegal fashion will not be provided on this website.