Remove flaps from the top of the box using scissors. Also remove the two bottom flaps on the longer side of the box. Leave the bottom flaps on the short side. You'll need these to tape your stage down to the table. You can trim them shorter, but leave them at least 3 to 4 inches wide.
Make sure the box is upright, flaps on the bottom. Cut out a window on one of the long sides of the box. The size of the window depends on the size of your puppets. Cut it large enough to show all the puppets that will be onstage at once. This is the front of your stage.
Cut down the sides of the back of the box to remove it, about 2 inches from the corners. This will serve as your backdrop (wall for background scenery). Trim the top of the backdrop so that it's slightly taller than your stage window.
Turn the back piece around and tape the sides to the sides of the front, creating your screen at the back of the stage. Leave enough room between the window and screen for the puppets to move around onstage.
Pull the back screen flush with the back edge of your table. Tape the side flaps down to the table.
Tape fabric or paper around the table from edges to floor, creating a skirt. This will hide the marionette operators' feet.
Cut out a window the same dimensions as your stage window from a large piece of colored cardboard or poster board to make your proscenium arch (stage frame). The top of the frame should be tall enough so the marionette operator's head does not stick up above it. Decorate the frame with paint, crayons and pencils. Glue or use double-sided tape to fix the proscenium arch onto the front of the stage, lining up the windows.
Cut holes in your box just behind the top of the window. Cut a piece of fabric for the stage curtain half an inch shorter than the length of the stage window, and about 1 inch wider. Tape the short end of the fabric to the dowel. Stick the ends of the dowel into the holes you've cut in the box, behind the stage window frame. Rolling the dowel back will lift the curtain, and rolling it forward will lower it.
Cut down the remaining poster board to the size of your stage window. Paint a different background scene on each one appropriate for your puppet show. You can draw anything from a meadow to a living room. Remember to keep the drawings simple, as you don't want to distract from the marionettes themselves.