In general I use Cesnna G1000 but I've tested the new autopilot in the regular version, 172 xp. It works as follows:
To turn it on, you must push "hdg" or "nav" button on the console. The autopilot starts working immediately.
To turn it off you must push the red button on the yoke, to the left.
If I remember correctly, on some forum it was said that the autopilot works in that way on the real plane.
I hope to be clear enough in my answer. English is my second language.