Usually I do a simple sketch of the shape of the head, like just a few lines to know where each element is. Then I always start with the eyes, I guess because it's the part I like the most. So I pass to the nose, the mouth, the face's shadows, etc. At last, I always draw the hair, don't ask me why I do it in this order. I just start and finish with the parts of the portrait I love the most - the hair and the eyes.
But actually, in the first image I haven't done any head's shape, lines or whatever to guide me, I just started by doing an eye. Guess I was lucky, and it went well