You'll find cartridge pleats on long gowns, such as graduation gowns and full skirts as well as on curtains and drapes. The pleats give drapes a nice fullness. Unlike boxed pleats or pinched pleats, cartridge pleats are soft and rounded. You don't press or iron cartridge pleats as you do others, as a result, the pleats form cylinders that jut out from the curtain rod. To get the fullness you want, you'll need a drape that is about three times as wide as your window.
Measure the length of your window from the curtain rod to the floor. Cut the drapery fabric so that it measures the length of the window plus 6 inches. Cut the lining fabric so that it measures the length of the window.
Place the drapery fabric on top of the lining fabric, right sides together. Align the fabrics along the top edge and pin together. Pin together along the two long sides as well.
Stitch the fabrics together along the top edge and the two sides, with a 1/2-inch seam allowance. Leave the bottom open. Turn the fabrics right side out.
Fold the bottom edge of the drapery fabric over 1 inch and press. Fold the bottom edge up 4 1/2 inches so that it meets the edge of the lining fabric. Pin to the lining fabric and stitch along the edge.
Adjust the stitch length on your sewing machine until it is at the longest stitch. Sew two rows of stitches along the top edge of the drape, about 1 inch from the top of the fabric. Space the rows about 1/2 inch apart. Leave 3-inch-long tails of thread on each end.
Cut the twill tape so that is as wide as your window and one third the width of the panel. Place the tape on the lining side of the drape, about 1 inch from the top. Line up the edge of the tape with one side of the drape.
Pull on the thread tails to gather the drape fabric to form the cartridge pleats. As you gather the fabric, pin it to the tape. Use one pin for each pleat.You want the fabric to fit perfectly between the tape when everything is pleated.
Hand sew each gathered pleat into place. Stitch only where the pins are.
Slide the drapery hooks around the tape and hang from the curtain rod.