To create rajas, or strips, char or roast the chiles, either by placing them under the broiler or directly on a grill or hot skillet. Roast for 6 to 9 minutes, turning every 3 to 4 minutes, until they are charred and blistered but not burned. Immediately place in a plastic bag; close the bag tightly and cover with a kitchen towel; this will facilitate skinning. One by one, remove each chili from the bag, peel off the skin and lightly rinse the chili with water. Cut out the stem and cut each pepper in half. Remove and discard the seeds, then cut the peppers into strips 1/2-inch wide and an inch long.