1. Measure your dormer to determine the size of the window seat. The one shown is 24 inches deep and 19 inches tall. If your house isn't absolutely square, measure the width of the dormer at the opening and again close to the window at 4 inches and 19 inches above the floor. Take the smaller width measurement and subtract 1/2 inch from it for the width of the window seat.
2. Cut and assemble the base from 2x4s as shown, using butt joints. Secure with glue and nails. The base should measure 12 inches smaller in width and depth than the size of the box that makes up the seat. The base lifts the box above the baseboard so you don't have to make special cuts to fit the box snugly against the wall.
3. From plywood, cut the top and bottom of the seat box to the measured dimensions. Cut the sides to fit flush. The sides of the window seat shown measure 24 inches wide by 14 inches tall (19 inches minus the height of the 2x4 base -- 3 1/2 inches -- and the 3/4-inch top and bottom panels).
4. For the lid, measure in 4 inches from each side of the top piece and mark. Drill a 1/2-inch hole in each corner. Use these holes to insert the saw blade and cut out the lid. Drill a 3/4-inch hole in the center front to lift the lid.
5. Screw four mending plates across the corners of the underside of the frame as shown.
6. On the opposite side, attach the lid with the piano hinge.
7. Glue and nail the sides, front and back together; glue and nail the top and bottom with 6d finish nails, and attach the box to the 2x4 base. Paint the window seat to match the walls.
8. To make the skirt, measure from the top of the seat to the floor and add 6 inches for hems. For the width, measure the width of the seat box and add 6 inches for the center pleat and 2 inches for side hems. Cut upholstery fabric to these measurements, then cut the panel in half. For the pleat insert, cut a strip of contrast fabric 7 inches wide and the same depth as the skirt.
Sew the contrast strip to the panel halves, using a 1/2-inch seam allowance. Make a 3-inch-deep pleat on each side of the center, placing the seams in the inside folds. Turn under the side edges of the panels 1/2 inch and hem. Cut a fabric strip 6 inches wide and 1/2 inch longer than the skirt width. Sew this strip across the top of the skirt, right sides facing, letting the strip extend 1/4 inch beyond each end. Press under 1/4 inch on each raw edge, then fold the strip in half and stitch. Use a staple gun to secure the strip to the plywood top. Make a box cushion, using a purchased pattern.