This portrait has a lot of very interesting features in terms of a learning experience. There is a very subtle overall shading pattern. Also to accurately recreate her hair as it goes from very dark to gradually showing more light and detail is a real challenge.
In the previous post is a video clip detailing the complete shading process for this portrait. It doesn’t show the drawing but just the shading which is a very similar process to painting a portrait. Starting with an underpainting and slowly building up the darker values.