Wipe off any dusty crevasses of the dried chiles using a damp paper towel. De-stem and de-seed the chiles, discarding the seeds and stems.
Preheat the oven to 400F.
Add the chile pieces to a baking sheet and roast for 1-2 minutes. When using fresh, pliable chiles you can give them the full 2 minutes. Older chiles that are less pliable will turn brittle quicker so I tend to give them a healthy minute in the oven and then take a peek at them.
Ideally your chiles are warm, fragrant, and still pliable after roasting. If they have turned brittle or crumbly they will take on a bitter flavor.
Alternatively, you can preheat a skillet or comal to medim-high heat. After de-stemming and de-seeding the chiles you can add the chile pieces to the skillet and use a spatula to press them down into the heat, giving them 15-30 seconds per side.
Note that most recipes will ask you to reconstitute your chiles after you roast them. I typically put them in a bowl and cover them with the hottest tap water available, letting them rehydrate for 20 minutes or so. More info on reconstituting dried chiles.