You need a curtain (or a material) that is at least half an inch wider than the door frame. You then cut out the opening for the stage. Country Living made theirs 18x16, but you can make it any size you like.
You then make "curtains" for the stage with a contrasting fabric. Cut out a square that is larger than the opening you cut before and then snip it in half. Pin the top of each piece to the back of the opening
If you want to make the fillip, cut triangles from a third fabric and pin them to the front of the opening at the top.Pin two 12-inch ribbon "tiebacks" to each side of the opening's front.
Sew the curtains, triangles, and tiebacks in place by stitching ribbon around the opening.
Sew a loop of ribbon onto the curtain's top corners, then hang from nails on the door frame.
The best part is when they are done, you can fold it up and slide it in a drawer. Or a box. Or under the bed. Just about anywhere!