Cut pieces of muslin that will cover the front and all sides of one wood frame.
Soak the muslin in water and wet it thoroughly. Remove it from the water and lay it flat.
Center the wood frame flat on the muslin and, starting at the bottom, wrap the muslin around the back of the frame and staple it to the frame with a staple gun. Once the bottom center is stapled, pull each side tight before stapling.
Continue working around the wood frame, pulling the muslin tight and stapling it to the back of the wood frame. Alternate which side of the center staples is being worked on so that the muslin stretches evenly and does not wrinkle or sag.
Attach braces made from 1-inch x 2-inch (1x2) boards with nails to keep the flats upright on the stage. These braces can be made by attaching a board one-third of the height of the frame to the bottom of the back of the frame at a 90-degree angle to the frame on each end of the frame and another board to both the frame and the back board on an angle.
Once the muslin has completely dried, coat it with primer or Gesso to prepare the muslin for painting. Let the primer or Gesso dry completely.
Paint the muslin flats to look like woods. Leave space between the trees for sky and to paint the ground at the bottom.
Repeat Steps 1 through 7 for each flat.
Draw a tree shape onto foam board with a pencil. Cut the shape out with a craft knife.
Build a brace for the tree with 1x2 boards. Cut one board two-thirds of the height of the tree and one board one-third of the height of the tree. Place these boards into an "L" shape and secure with nails. Nail another board between the end of the shorter board and the longer board on an angle. The brace should be shaped like a triangle.
Attach the tree shape to the wood brace with nails through the front of the tree.
Paint the front of the tree shape to look like a tree.
Repeat Steps 1 through 4 for all of the trees required.