We have the Base Colors all in: the skin color, the necklace, the hair, the flower. This is how everything looks:

TIP: I added a plan white background, so create a layer and fill it with a color that you like. Also remember to create a base color for the clothing.

I am going to hide all the layers again because I am going to add Shadows. In order to add shadows, you must understand the concept of Color which is:

Darker Colors create Shadows - Something we all know. This is how I created the Shadow for the Chin, you may follow.
TIP: I used the PEN TOOL to create this as well.

Now that you see what shadows look like.. try adding darker shades to the hair as well.

TIP: SOME PEOPLE go extreme when it comes to shading, to create that process, all you have to do it take your time and create tiny lines from light to dark, with my style, I simply add tiny shapes where and when needed.

Highlights are the reverse of this process by getting lighter colors, don't be afraid to be BOLD.