Part III - Skin and face.
The plan is to do another one on how to add texture to the skin. That will require another sample image, though, because this one is much too small to show the details properly. It'll come in time, though.
Download if too small to see.
Part I : Clouds, Trees and Technique
Part II: Marble, Stone, and Cracks
Part III: Skin and Face
Part IV: Details, and Clothes
Part V: HairHow to Learn from Tracing Final Image