Buy wide muslin. It is available in 108-inch widths by the yard. Sew a pocket across the top, so it can be hung on a line of rope or secured to an existing stage curtain with industrial strength Velcro.
Dye muslin to create a monotone backdrop. Prepare your fabric by washing and drying it. Rit dye works well for the purpose. Follow the package directions to complete the process. For a mottled background, use several shades of the same color paint. Apply the paint with a sponge in a random pattern. Cover all of the fabric with equal amounts of paint.
Create a scenic backdrop for plays and performances with help from artistic friends or students. Choose a picture that can be easily recreated. Project it onto your fabric and trace around the contours until you have captured a recognizable drawing. Use acrylic paint to fill in the rest of the picture. Finish with a matte varnish.