This is a very good video from Sean Armenta created for the website Fstoppers about using the Clone Stamp and Blending Modes.
Fstoppers is an excellent site with a lot of good info about photography and computers.
Here's another way to correct skin tones by Aaron Nace
Shirley Cards for adjusting Kodak color printing machines.
This technique uses Gradient Maps and Blend If... to enhance skin tones.
This video is by Prince Meyson