Place the chile directly over an open flame or under a broiler and turn until blistered and charred. Do this quickly so that the flesh does not cook. Do not attempt to char a chile on a fork over heat; you’ll be at it all day.
If your chiles are wrinkled and not too fresh or if you are using a broiler, rub the skins lightly with vegetable oil before charring.
Immediately put the charred chiles into a plastic or paper bag or under a damp cloth and set aside to sweat for about 10 minutes. Do not rinse them in cold water as some cookbooks suggest; they will lose flavor.
If you have sensitive skin, wear thin rubber or surgical gloves before you start this step. Slide your hands over the chiles and slip off the blackened skin. Don’t worry about picking off each bit of skin; it will take forever.