Hi Theresa,
The
Backstage Magic Tour is my all time favorite Disney Tour!!! You're going to love it!
Here's what I'd do. From the Polynesian Resort, I'd walk to the Transportation and Ticket Center (TTC). Its a short walk - less than 15 minutes. You could also take the Resort Monorail around to TTC but its the last stop on the loop and may take you 25 minutes or so depending on whether or not you get stuck in holding patterns (I'd walk if your party is able).
From TTC, hop on the Epcot monorail. Its about a 10 - 15 minute ride to Epcot. When you walk down the ramp from the monorail, you'll go through security but not through the gates to enter Epcot. Tour groups meet near the Guest Relations location outside the gates.
If you are at the Epcot monorail station at TTC by 8 a.m., you'll have plenty of time to make it by 8:45 a.m. The Backstage Magic Tour ends in the Epcot parking lot near where it began. At the end of the tour, you can hop back on the monorail and head back to the Polynesian.
Because you are going in December, the morning may be a little chilly but it may warm up nicely in the afternoon. Feel free to wear a jacket. You can leave it on the tour bus in between your stops once it warms up and not have to worry about carrying it around all day.
Wishing you minnie magical memories :-) Allison