Try on the cape. If your neck closure on the cape is a clasp, make sure it fits snugly. If the closure is a drawstring, adjust the neckline to where you feel most comfortable.
Drape the extra fabric, which should be at least a 3-by-3-foot square, over your head to simulate a loose hood in the style of the "Phantom of the Opera." Place the sewing pins along the bottom of the fabric where the seam will be once it is sewn into the neckline of the cape itself. When you are finished placing the pins, they should simulate the cape's neckline.
Cut out the hood fabric according to the placement of the pins. Keep the fabric scissors as close to the sewing pins as possible as you cut along the neckline.
Choose a side of the cape's neckline to start on. Accommodate a 2-inch seam allowance on the hood as you sew it onto the cape. Use a French seam, which requires you to sew the two pieces together with the wrong sides facing each other, to hide the raw edges of the hood inside the cape. Once completed, trim the excess fabric along the inside of the cape where the seam connecting the hood to the rest of the cape lies.