Chiles are roasted in order to remove the thin skin that covers them, as well as to give them a unique, rich flavor. There are several methods, but the most commonly used technique is roasting over direct heat.
Place the chiles directly over a medium flame on a gas stove. Turn the chiles with tongs until their entire skins are charred, approximately 5 to 10 minutes, depending on the heat of the flame and the size of the chiles. Do not char the chiles too much or the flesh will burn and taste bitter.
Immediately place the chiles in a plastic bag and close the bag or cover the chiles with a damp cloth and leave for 10 to 15 minutes. This procedure is called “letting the chiles sweat,” and it serves two functions: to make the thin skin easier to remove and to let the chiles cook slightly in their own steam.