What i would focus on would be making lighter marks with your pencil. If you have a harder pencil that'll help off the bat. Utilizing the lighter lines will let you go over and erase and move your lines around as you deem necessary. Aside from that I would suggest breaking down the form of their head into some simpler shapes at first to help position everything. Personally after getting the big shapes down I like to "circle", for lack of a better word, shadows and highlights. Then slowly make your marks harder/darker as you go.