Heres where I usually stop to draw in the features, starting with the eyes. Ill usually draw a perfectly round pupil, then draw the shape of the eye around it, using sharply angled lines that Ill soften and round out later. Those extra sketched lines are guidelines:
First, draw a vertical line bisecting the head from the point of the chin to the crown of the head; its going to arc a little, but its supposed to. If the head is tilted instead of looking straight forward, youll have to estimate a little but it wont take you long to develop an eye for it.
The head is basically composed of a circle with a triangle appended to the lower half; in order to draw the guideline for the eyes, look at the circle composing the upper half of the head, and sketch a line that would divide the circle into equal hemispheres, following the circles arc. Your eyes should be centered so that the arc bisects them.
The generic way to position the nose is to draw another arcing horizontal guideline at a point halfway between the guideline for the eyes, and the point of the chin. Where the horizontal guideline bisects the vertical guideline is where the center point of the nose will rest.
The positioning of the mouth is similar: draw another bisecting line halfway between the noses guideline and the chin. This line is the crease/part of the lips, with upper lip above, lower lip below, while the vertical line neatly divides the mouth into two halves.