You could take a look on this 757 tutorial, showing very similar steps regarding the operation of the plane. http://captainsim.org/yabb2/YaBB.pl?num=1220997876 And I saw that Youtube features a lot of 757 (and 767) related videos, perhaps those can help with getting to know some steps.

Regarding the actual autopilot operation, the manuals (http://www.captainsim.com/products/b767/manuals.html) should give you a precise description of each button and mode. So I think the rest comes down to performing a little trial and error then.

The default FSX planes use a very basic kind of autopilot, so, unfortunately, you may not be able to really apply much of the former knowledge since the 767 installation is far more sophisticated. Well, once you have the FMC of yours knowing about the flight plan and some basic data, you are good to go. So mind the mentioned tutorial to set it up and sort of unlock all the features of the autopilot.