1) Start by clicking on the "Channel" you want to use
2) Click "Preview" on top right
3) Verify if your content looks correct.  If part of the content is offscreen, you need to change to "Fit to Zone"
4) Click on Channel again
5) Pull down the option that says "Fill Zone" and change it to "Fit to Zone"
6) Use "Preview" to verify that your content looks correct
7) Hit "Publish" to make your changes go live on the screens.