There's not much to it. Just put the dried chiles in a bowl with enough hot water to cover them. Put a saucer or an upside down coffee mug on top to keep the peppers submerged. Leave them there until they are soft. It usually takes from 15-30 minutes.
Chipotles are very hard and tend to take longer.
You can speed up the process by simmering them gently in water on the stove top. If you're making stuffed peppers, be sure to take the chiles out of the water before they get too soft to stuff. If you're making a chile puree, you want the peppers very soft, so you can leave them in the water a little longer. When they are soft enough, pull the stems off and scoop out the seeds. Put the flesh in the blender with enough of the soaking water to get the blades turning.