i usually start by mine by drawing in the eyes. positioning them is crucial, cuz if you drawing them slightly sideway, it'll throw off the whole face. i then mold the facial lines around the eyes to a point where i then fit in the nose and mouth (which is a simple line for the mouth and a v for the nose lol). after that, the hair comes in next. it could either be simple or if you want, add in lots of individual hair strands.
its a trial and error process, but once you have your final look, ink it. some second opinions would help. references help too, if you're not sure how to position your head or need some inspiration on hair.
well, that works for me. hope that helps some. good luck on your sketch