How to do it
Determine the size you want the shade to be; add 4 inches to width and 8 1/2 inches to length. Cut shade and lining fabric to size.
Lay fabric right side down. Fold and press 2-inch hems on long sides and base. Unfold. Fold bottom corners; press. Refold hems. Slip-stitch corners, mitering them and leaving a 1/4-inch opening on one side for a dowel. Hem sides and base by hand (or by machine if you don't mind visible stitches).
Lay lining right side down. Fold and press 2 3/8-inch hems on long sides and base. Place lining on shade, wrong sides facing, so shade shows on long sides and base. Slip-stitch lining to shade on hemmed sides.
Determine positions of dowel pockets, and mark them on back of shade with a disappearing-ink pen. Pockets should be evenly spaced at intervals of 8 to 12 inches (depending on how wide you want the pleats to be). To determine position of bottom pocket, divide size of intervals by 2, and add 1 (for example, if you're spacing dowels 12 inches apart, the bottom pocket should be 7 inches from the bottom of the shade). The top pocket should be at least 10 inches from the top. Begin measuring and marking your intervals from the bottom, and don't worry about the precise size of the top interval. Sew cotton strips as shown: Fold in half lengthwise, wrong sides together; press. Machine-stitch a 3/8-inch double hem on the long edge. At one end, fold, press, and machine-stitch a 3/8-inch double hem.
Pin folded long edges of pockets along pen marks; machine-stitch. Insert dowels in pockets, and slip-stitch a 3/8-inch double hem. Insert bottom dowel into the mitered corner's opening. Close with a slip stitch.
Draw a line 6 1/2 inches below top of shade. Wrap batten ends in fabric swatches, and staple. Position batten as pictured; staple in place. Roll and staple batten until edge covers marker line.
Stitch rings to hemmed edge of pockets, forming 3 columns: one up center and one 2 inches from each long side. Attach cord lock (to merge cords) to batten 1/4 inch in from its edge on side where strings will fall. Attach a screw eye to batten at top of remaining ring rows. In each row, starting at bottom of shade, tie the end of a length of cord to lowest ring. Thread cord through rings in that row, through screw eyes at top, then through cord lock. Thread loose ends through cord condenser; knot below, trim 2 of the cords, and pull condenser down to cover knot. Cut remaining cord so that it hangs 2/3 of the way down the shade. Thread through tassel and tie knot to secure. Mount shade with appropriate hardware.