Think about the bones of the face now. This will probably be simpler if you adjust the layer opacity of your lineart so that you can see through it with relative ease. I also suggest opening a new layer and choosing a new color to do this next step in. That way, you can see differentiation in your drawing process
how do the cheeks sit? the jaw? the forehead? What gives the face dimension? Done be afraid to drop shadows in here to help you see the contours independent of the photo











