In this tutorial I take a landscape picture and want to darken the sky. A building is in the picture, so simply masking the sky for selection can cause complications. Instead, we can use the shadows and highlights feature in Photoshop to edit the sky. I also show how to make a selection using painted masks. This way, our edits made with highlights/shadows is only going to affect what we want darkened.

Picture can be seen here:
http://flickr.com/photos/towert7/2657944750/