Thanks, everyone.
I painted the backdrop with some satin latex paint I got at Home Depot. The color for the sky is Horizon Haze and the lower wall I painted Green Grass. These were Behr colors that I picked out by comparing with photos.
I painted the clouds using Gavin Miller's techniques:
http://www.the-gauge.com/showthread.php?t=2906
I cut some stencils from cardstock and used a spraycan of flat white paint.
For the trees, I used these stamps and acrylic paints.
I worked about a foot and a half section at a time. I would stamp a tree at each end of the section that I was working on. Then I used a light green that I mixed on my palette to paint a background color over the blue. I painted it about 3/4 the height of the two trees that I stamped for guides. I did this using a wide artist's brush.
I used a few different shades of green, evergreen, forest green, and woodland green, for the trees. I mixed these three colors together, as well, to make a few in between shades.
I used the widest of my artist's brushes to paint the paint directly onto the stamp. Not real heavy or it just blobs. I just experimented with how much to put on the stamp until it looked right.
I then stamped the trees onto the backdrop. I worked down the section, repainting the stamp about every two trees. Then I went back and touched them up by hand and painted in the trunks with burnt umber. Then I painted in under the trees with a grayish green color that I mixed on my palette.