Ok ... Confession time. I shoot manually and I am not always perfect. Sometimes I over or under expose. But I usually am watchful and catch myself. In this case I am going to show you a fix for an under exposed image as well as a skin softening technique that I use. To start off here is the before photo:
The first step I took was to adjust the levels
Here is where to find the levels adjustment slider
And here is my adjustment for this particular photo, but each one is different, so play around with each of the sliders to get the best adjustment.
Now on to skin softening.
The first thing to do is make a copy of the background layer so I have 2 identical layers, then the top layer I add a mask to it (that is a the little box with the circle in it at the bottom of the layers pallet) then you will see a white box next to the layer fill that with black. Then click on the image layer and go up to Filter > Blur > Gaussian Blur then go ahead and blur it until the skin looks smooth. Click apply. then go and select your mask layer and then select a large soft paint brush and the color white and start painting on areas of the skin you would like smooth and soft. If it appears over photoshopped when done go ahead and adjust the opacity down on the layer, then flatten and then your done.
Here is the final product: