Call furniture stores and department stores. Request the largest cardboard boxes they will give to you.
Determine the height and width of the backdrop needed for your stage. Lay out flattened cardboard boxes on a flat surface. Fit the edges of the boxes together, trimming with a box cutter when necessary, to make them fit. Tape the boxes together using wide masking tape. Trim the outer edge of your assembled boxes to the dimensions of your planned backdrop.
Draw the desired scene on the cardboard, using a carpenter's pencil. Starting with the top of the sky, paint the picture using liquid tempera paint. Block in the sky and the ground, then add details such as trees, bushes, fields of wheat and so on. Allow it to dry.
Stack other boxes into two or three stable columns. Weight the bottom boxes with sand bags or concrete blocks. When the paint has dried, stand the cardboard flat up against the columns of boxes.
Poke holes through the flat and into the back of the boxes using a screw driver. Fasten the flat to the boxes using twist ties, long paper brads or pieces of string. Paint over the spots where the ties go through with paint that matches the scene.
.Position your cardboard flat at the back of the stage. The scenery is now ready for the show to go on!