As always, I start with the eyes, and I recommend you do the same. Starting with the eyes has advantages, if you're not absolutely sure about your sketch and how the shading part with look, starting with the eyes is the best idea on a portrait since the eyes are one of the main centers of distraction. That's were people look first, or teeth if they're visible , but anyways, you need to make sure your eyes look good, then you can change the sketch around them . This is not particularly the case, since in this picture the eyes are barely visible .