I first needed to line-up the green screen footage layer with the background layers used in this composite, so I set a guide on the edge of the door and moved/rotated the green screen footage to line up the edge of the blue panel with the guide. I then create a large feathered mask around the live action portion of the screen footage as a “garbage matte”.

I then use the Keylight keyer filter on the green background and fine-tune the settings to get the best possible key of the Santa character. (Oooh… he looks a little pissed here, doesn’t he?)

Next I use the Keylight filter on the blue foreground panel and adjust to accommodate for the shadows from his hands, so the door shows through.

Lastly, I use the Levels filter to adjust the color of the keyed character to attain the best color and lighting for the composited scene.

You can see the before/after video of this process in the movie link below:
NOTE: YouTube screws up the first few seconds by over-compressing the video, but it clears up – just let it keep running!

To see this tutorial in step-by-step detail, you can get my new training DVD here: “Learning After Effects CS4″ from PhotoshopCAFE.com