Lets draw an eye :D

the eye of the tigress by BA_Baracus (Pencil HB 6B and 8B + eraser)

Start with the outlines and dimensions (measure & measure)

Put in the darkest area's

Fade out the darkest area's and check for improvement.
Add whiteness with eraser on the lightest area's

Result after using HB-2B pencil

Add darkness with a 6B-8B pencil, or even charcoal

Keep adjust look for area's that are not quite right.
And voilá we got ourselfs and eye!