I painted this backdrop for an O-scale layout over three weekends. The sky was already painted with acrylic. All the paints I used were also acrylics. I painted the clouds with a very soft brush. Then used a sponge brush to paint the hills.
After those were dry, I used triangular tree shapes torn from corkboard (the same as was used under the tracks) to make tree stamps. I dipped these into paint or painted thickly onto them and then stamped the shapes onto the hills. Then I painted some more detailed trees and bushes. I tried always to keep one side darker to give the impression of the sun shining from the side.