First,
draw a rough oval in the general shape of the eye, then draw in a
circle with part of the top reaching above the top curve,
adjusting the position to indicate the direction the subject is
looking (in this case, straight ahead).

Next, draw a curve for the eyelid, using the top of the
circle as a rough guide. Erase the guideline
.
Deciding on an eye
color, fill in the iris with a light
version of the color, and draw a line of dark color across the
top of the iris - this gives the eye the illusion of depth and
3-dimensionality. Then fill in the pupil with a very dark color or black before adding light-reflections as highlights. I find
that it gives more realism if the light reflections go across the
pupil and the iris.
Finally, add eyelashes, and
sharpen up details. I have given two samples here as there are a
wide variety of techniques, which make a big impact on the final
mood of the illustration.