This detailed tutorial plus a Photoshop action to simplify the layer setup will get you going quickly.

Click HERE (http://www.retouchpro.com/forums/showthread.php?s=&postid=54679#post54679).

Pictures worth 1000 words...

Example 1:
Before (http://www.retouchpro.com/forums/attachment.php?s=&postid=49544)
After (http://www.retouchpro.com/forums/attachment.php?s=&postid=54678)

~DannyR~