Buy the dried ancho (a.k.a. poblano) in the spices or Mexican section of your supermarket. DO NOT attempt this with fresh poblanos you damn fool. Do you want a spice or diced green chilies? De-seed and de-stem all your dried peppers. They have the texture of a weather worn fruit roll-up so the easiest way to get them open is make a slice down the side (as seen above).
I didn't take a picture but you want to toast your chilies flat on a cast iron skillet for about 5 sec per side. Now dice them up before you put them in the food processor genius.