The most difficult part is the face. As in the drawing tutorial, we need to push lines where we want them.
Shading helps push things where they need to go. Concentrate on the eyes, nose, mouth, and cheeks. If the cheeks look too thin, erase into the hair to widen them. Are the eyes too small? Shade outward to make them larger. Use your pencil and eraser on equal terms... they are both drawing tools.