Preparing chiles for stuffing
Here is the method to prepare any fresh chile for stuffing.
Fresh chiles like jalapeño and poblano are hot, and the seeds and veins can irritate skin. It is advisable to use food preparation gloves when handling the chiles, especially when de-seeding and de-veining them. Make sure you wash your hands after handling and before touching your eyes.
Broil chiles in an oven broiler or heat them in an ungreased griddle or pan until they are blistered all over. It's alright if they char a bit, but take care that they don't get too charred, or the flesh will be soggy and will stick to the skin. (The way our cook would char them was to place them directly over a medium flame in a gas stove and keep turning them around.)
Place the blistered Chiles in a plastic bag and let them "sweat" for 10-15 minutes. Let them cool down, then peel off the skins. This should be fairly easy, except perhaps where the chiles weren't blistered. I like the chiles to be totally skinless, but if the skin is stuck to the chile (where it wasn't blistered), I won't worry too much about leaving it, especially if it would tear the chile.
For each chile, look for a place where the flesh is open; if there is no such place, make a vertical slit. Either way, ensure that there is an opening from almost the top to before the tip of the chile, but do not open the chile all the way to the tip. Using this opening, remove the seeds and the veins that the seeds are attached to. This is what gives the chile its heat. The better you remove the seeds and veins, the less likely you will have a hot chile. Take care when removing the vein -- if you just pull them out, it is likely to shred the chile to ribbons. Using a paring knife, cut the vein (and the membrane that holds it) carefully. Rinse the chiles in running cold water to remove any leftover seeds, then pat the chiles dry.