Meanwhile roast the chiles. There are lots of ways to do this: you can broil them in the oven or char them over a gas stove. (Here's a resource)
I grilled mine outside over medium heat. It took 12-15 minutes total, turning the chiles every 5 minutes or so. However you choose to do it, just make sure that the skins get nice and black. Afterward make sure you steam them for at least 20 minutes. You can do this by placing a bowl over a plate of hot chilies (with no escape for steam), closing them in a bag, or putting plastic over the bowl they are in.