This video is part of my Scenery - Making it Real clinic. In it I show my technique for:
Using an airbrush to paint clouds. I like to keep my clouds amorphous, that is with out distinct shapes. This makes them perspective neutral so they look OK even when viewed from off to the side.
Painting distant hills and topping them with a tree line or clear cut.
I also discuss the order in which I paint backdrops and build foreground scenery.