Perhaps someone else can direct you to a general "how-to" thread, because i'm not too familiar with the MR forum.

For this deal though, you just have to navigate your browser to:
www.aircanada.com

When searching for a flight, choose the option for "multi-city" - this is located in underlined red font immediately next to the pull-down choices for roundtrip/one-way searches.

Enter each leg of the itinerary for the cities you want, choosing dates in late Aug through Nov.

I think people usually recommend that you look up fare availability in the required booking class beforehand, so you don't just keep trying at AC's website, which is tedious and time-consuming. In this case the booking class is "E", so if that is available, you should get the ~$800 price.

I just did the JFK-YVR-HKG-YVR-JFK search for 10 Nov 08 - 18 Nov 08. The price was $852.67 all-in for ELXAPHK base fare.

Good luck.