For the vast majority of people (meaning, those without FFLs and SOTs), the only way to make an AR15 full auto legally is to purchase a registered Drop-In Auto Sear (DIAS).  The DIAS is itself considered a machine gun, even though its just a couple of chunks of metal.  The process for buying one of these is the same as buying any other machine gun, but they're a bit cheaper.

You can then buy a post-ban AR lower, have the "block" milled out, install the full-auto fire-control parts, and drop an upper (of any configuration) on it.  Note that you must leave the DIAS in the gun unless the rifle is first put back into legal post-ban configuration.

Otherwise, it is not legal to convert an existing AR to full-auto unless you're a licensed manufacturer and you're making them for a government agency.  No new *transferable* "machine guns" have been allowed on the registry since 1986.

-Troy