The easiest way is to buy the widest fabric you can find, take a measurement from the back of your neck to the floor, and cut out a half-circle with your measurement as the radius. Add a hood and hem the edges. If you want a really big cloak, make two half circles and sew them together along one side, then finish as above. If you want a lined cloak, make two identical cloaks as above, then put the right sides together and sew along the outside edges, then turn inside-out. If you'd rather have a commercial pattern, the Folkwear Moroccan Burnoose pattern makes a great cloak if you don't stitch it closed in the front.