After you have set up your processing pipeline and your media, you want to set up the viewports for keying.
When keying, you want to set the viewport to 2-up view, to display the schematic and the result views simultaneously, enabling you to view the result as you work.
To set up the viewports for keying:
The view area is split in two. The schematic view is displayed on the left, by default.
A white frame appears around the viewer to show that it is selected.
The result of your key, in its current state, is displayed in the right-side viewer.