First, roast or char them
To do so, you can either place them on a tray under the broiler, directly on the grill or directly on the open flame, which is what is typically done in Mexico. I prefer to broil them because you can do many more at one time and just seems faster and easier.
Whatever method you choose, turn them every 2 to 3 minutes for a total of 6 to 9 minutes. They must seem charred and blistered on the outside but the flesh must be cooked but not burnt. Just like roasting marshmallows in a fire.
Second, make them sweat.
Once charred and hot place them in a plastic bag, close it tightly and let them sweat for 10 to 20 minutes.
Third, peel and rinse.
Preferably under a thin stream of cold water, remove the charred skin which should come right off. Make a slit down one side of the pepper and remove the cluster of seeds and veins.
If being used for stuffing, keep them whole with the stem. If being used for rajas, take the stem off and make slices. I like them of about ½ inch wide.
If you want your Poblanos to be fairly mild, once prepared, let them soak in water with salt for about 10 to 30 minutes and then drain.