Measure the length and width of the curtains with the tape measure to determine the amount of lining yardage needed.
Cut the lining to the same size as the curtain. Pin and sew the lining fabric to the curtain, right sides of the fabrics facing each other, with a straight seam around the curtain edge at the hem lines, leaving a 6-inch opening, with the sewing machine. Trim any excess fabric to 1/4-inch.
Turn the curtain to the right side through the 6-inch opening and hand stitch the opening closed with the needle and thread.
Pin and sew the pleating tape to the top edge of the lining, 1/4-inch from each long edge. Pull the pleating cords to pleat the lined box pleated curtain.