Draw a 10 by 14-inch rounded window template and a 14 by 36-inch rectangle on the cardboard and cut them out with a utility knife. Take the templates with you to buy the wood to build your puppet stage.
Purchase your wood from a home improvement store or lumberyard that offers custom cutting. Most retailers charge only a nominal fee; be sure you call ahead to check pricing and avoid peak business hours. The service person is likely to be more accommodating when there is no crush of customers demanding attention.
Sand the window and stage openings cut in plywood; paint all your wooden pieces and set them aside to dry. Use chalkboard paint on the rectangle cut out to make a marquee for upcoming puppet shows.
Glue an 11-by-3/4-by-3/8-inch strip of pine to each of the narrow plywood boards so it sticks up a ¼ -inch above the windowsill. This prevents the puppet stage window from swinging in.
Attach the stage walls to the front with 2-inch hinges. Suspend a length of chain between screw eyes, at floor level, to prevent the sides from opening more than 120 degrees.
Install the window boards with 1-inch hinges on the outside and glue pull knobs on both sides of each window to open and close them.
Cut your velvet in half to form two 2-foot by 3-foot rectangles and sew a 5/8–inch hem on the sides and bottom. Turn the top down 2 ½ inches and stitch to make a pocket for the curtain rod. Insert the rod into your curtains and attach it to your puppet stage with cloth loops thumb tacked to the front wall. You are ready to raise the curtain on your first puppet show!