Use your plumb-line both vertically and horizontally, like a moveable grid line, to accurately relate and place the lenses.

I prefer to use conté as a painterly medium. This entails blocking-in and stumping the darker tones (Additive) followed by lifting out the light tones with painterly strokes of a kneaded eraser (Subtractive).

Focusing within the orbit of the eye I begin the additive/subtractive process of constructing form. This is not for the faint hearted; this stage always gives me heart palpitations. This radical step forward is necessary. One could take a more cautious or timid approach but the result could also be overly cautious and precious. Better, I think, to engage in a measured attack, neither too little nor too much. Experience will teach you the balance to strive for. Be sure, though, to stump in with your finger. A tortillon will dull the conté at this juncture.

The abstract shapes of light and dark can now be resolved by painting out with your kneaded eraser. I am not thinking eye here, instead I am concerned with how each shape of dark and each shape of light relate to the other.

Once the respective shapes have been satisfied their primitive tonal values can be indicated. What I mean by primitive is that you only want a rough approximation of the relative values of light to dark. Given the scarcity of plastic information in the drawing thus far you do not want to fully commit yourself and work up the orbit of the eye to its full tonal resolution. I am still working somewhat generally here.