Measure the high and low points of the angled opening under the stairs, and measure the length along the floor as well. Mark one quilt or wide fabric piece on the "wrong" side. Use a finished hem or selvage for the bottom hem and leave 4 inches at the top for a folded-over curtain rod sleeve. Press the raw edge for the sleeve under ½ inch, fold top edge down 1 ¾ inches and use a sewing machine to stitch the sleeve.
Mark a four-paned "window" on the quilt with paper tape. Stitch the quilt around the outside border of the window, using the tape as a guide. Cut out the square for the window, being careful not to cut the stitching that reinforces the opening. Sew wide grosgrain ribbon around the square to frame it on each side. Measure the opening and cut ribbon to sew together in a cross or plus sign, slightly larger than the opening. Sew the open edges of the frame ribbon together, catching the crossed ribbon in the stitching to make the window frame and open panes.
Slip the curtain rod into the sleeve. Screw two curtain-rod hooks into the wood frame under the stairs and hang the quilt "wall" in the opening.
Prepare a second quilt or length of fabric to cover the opening under the stairs that spans the width of the staircase. Sew a sleeve straight across the top of the fabric for the curtain rod. Measure and mark a 48-inch-tall, 30-inch-wide rectangle for a door right in the center of the fabric, from the floor up. The measurement for the door can be adjusted to suit the size of the fabric panel. Stitch around the door edge, cut out the door and frame the opening, front and back, with grosgrain ribbon.
Cut down two sheer curtains to 50 inches long, for a 48-inch high doorway, and gather the top edges on the sewing machine. Pin the gathered edges on the inside of the quilt "door" so they meet across the top. Stitch them in place and remove the pins. The sheers will hang in the doorway, split in the center for easy access to the playhouse.
Screw curtain-rod hooks to the underside of the stairs, slip the rod into the door panel and hang it up. Put a couple of beanbag chairs or some big cushions in the playhouse with a basket of stuffed animals or favorite toys. Embellish the playhouse with an area rug, crayon masterpieces taped to the back wall and a string of fairy lights suspended from the underside of the stairs.