I do the highlights with the same tool I smudge! The ’retouch’ tool. Only the retouch mode is set to »lighten RGB. Figures… Again I can’t tell you the exact size, hardnes, bla bla bla. You have to try it – how it fits you. OK! I’ve done that! Here it is…
HINT: Again, don`t overdo it, don`t highlight too much. And offcourse! Highlight on the places that need to be highlighted – same with shadows.
My second technique to do shadows: I dont choose the lighten RGB in choose the darken RGB! It works the same way how you do highlights. I shadowed her neck that way.
Alright, the neck is done. And now, my favorite part –THE EYE! I love eyes – so small and yet full of details and character.
I cleaned out the pencil doodles and I drew two lines with the airbrush. Above and under the eye. Folowing: Some highlights here some shadows there 
As shown in the picture.
