How do you make easy backdrops for easy plays?

Remember that its the action stage front that is the most important. Backdrops are simply adding atmosphere.
If you have a plain wall then hang white cotton sheets against it - use lighting as a down flood in 100% Blue to give a blue sky effect, you can place strategically placed floor floods with green gels in and close the apertures down so that the light comes about 25% up the Blue wall -t will give a green field/blue sky effect
Place some hardboard painted trees and there you have a 3d effect for a forest looking out on a field.
By using the white backdrop it will reflect the colours you bounce really well. You can also use dust sheets or dust sheets with white emulsion on that will keep cost down.
If you are using low from lighting and have some overhead projectors you can throw a scene on the rear of a white gauze (back projection) - Pictures taken from the Internet and colour printed to acetate will do a grans job - watch wording so that you mirror effect the letters or they will be reversed.