Next, i'll add more dark shades on her left area. Using the Pen Tool (P) for a more precise outline, I isolated the area in which I'll be working on. I followed the outline of her nose to give it more detail, but as you can see, I traced a bit outside the outline of her face and nose. I'll explain why when I get to it.

Apple+Click (Mac) or Right click (Win) to bring out the options below and select 'Make Selection'.

From the left you can see that I opted to feather the selection by 12 pixels. This would make the 'mask' I'm creating to have a softer edge to compliment the airbrush look of coloring that I do.

You can vary your Feather Radius depending on how sharp you want your shadows to look on your skin.