Remove one of the flaps from the top of the box on the long side, using scissors or a razor blade. Then turn the box upside down and cut out a large window. Make sure to leave ample room for the puppets to move around, since this is the window for your stage.
Construct a lip or ledge for the puppeteers' wrists to rest on by cutting strips of cardboard (about 2 to 3 inches wide) from the flap you removed earlier. Cut the strips the same length as the sides of your stage window.
Run a thin line of glue along the edge of one of the strips and attach it to the stage window from inside the box. You want the ledges to stick out on the inside of the box, where the puppets will go, not on the side the audience will see. Make sure to line up the edge flush with the window opening, since you want a nice, smooth, even ledge. Repeat this process on the other 3 sides and let dry for about five minutes.
Lay the box on its side so the stage window is facing the audience, and the back flaps are facing the puppeteers. The one flap left on the long side should now be on the bottom. Tape this flap to the table to ensure a steady stage. Take the back side flaps and tape only the top and bottom corners of them together, closing up the back of the stage but leaving just enough room for little hand puppets to fit through.
Paint, color, or draw on your stage as you like. Stencils are good for writing the name of your new stage above the main window. Glitter and some gold crayon can produce a proscenium (stage frame) effect.
Cut your fabric the length from the table top to the floor, and make sure it's wide enough to go around the front and sides of the table. Once you cut the fabric, tape it so it drapes around the table, hiding the puppeteers.
Paint fabric scraps or draw on extra cardboard pieces to create fun and simple backdrops (scene backgrounds) you can easily tape to the back of the stage.