Draw your scene on paper in as much detail as you can, and color it using a medium such as watercolor paints or markers to indicate which colors will go where. Choose a photo or other image if you cannot draw what you want to your satisfaction. Choose an image in the same height-to-width scale as your backdrop or scenic panel if possible.
Trace the image in a clear, black outline onto another sheet of paper, including all of the major details to create an outline template for transfer to the larger canvas of your backdrop or scenic panels.
Set up your backdrop or scenic flats against a wall. Set up an opaque projector, available at art and hobby stores, to project your image onto your backdrop. Place the outline image on the projector bed, and turn it on. Adjust the distance of the projector from the backdrop to size the image.
Darken your work space as much as possible to make the projected image as clear as you can. Use charcoal pencils to outline the projected template onto your backdrop. Stay as true to the original as possible. Consult the colored image to draw in details where necessary.
Mix 4 parts of latex paint to one part water in a bucket so the binders will penetrate the surface of the backdrop rather than peel off from the weight of the paint. Work with un-thinned paints if you prefer on hard-surface backdrops or flats.
Block in large areas of color with a wide, long-handled art brush. Work from the top of the piece down, so that any drips can be corrected by painting over them. Paint each section of the backdrop with a base color, which is the color most dominant in that area.
Add texture to each section as needed. Paint bark on trees, speckle stone and brick with slight variations of the base color mixed with white or black for contrast. Try to add two more color tones to each area to give depth to the image.
Determine where the light is coming from in your image to position the highlights and shadows. For example, if the light is overhead to the left, shadows go under and to the right of objects in the scene.
Add shadows by mixing a small amount of black into each base color. Thin this paint a little more so the background shows through and the shadows appear on top of the painting. Apply shadows on the bottom and opposite side from the light source in every area of the painting. Apply highlights in much the same way, mixing white into the base colors. Apply highlights on the top and side facing the light of each object.