Before any of the scenes can be animated, they must be drawn by the Storyboard department. In the years since this storyboard was made, the storyboard department has switched to drawing them digitally.
Before new characters appear in an episode, animators make "Corels" of them which must be approved by Trey.
Here's another "Corel", this one of Cartman. Every single new pose and outfit must be created and approved.
Cartman got a total character make-over for the last scene.