Start out with a basic sketch of a circle. The square makes it easier to sketch the cicle. Use an F or HB pencil.

Sketch in the iris and the outer eye.

Use a blending stump over the whole drawing to soften your lines.

Use an eraser to erase the highlights in the eye and erase the lines that you don't need.

Use a soft pencil to sketch in the iris and the dark lines. 4B work great for this but 3B and 2B can work also.

Go over your rough shading with a harder pencil. I used an F pencil but HB pencil would also work.

Define the eye by blending in your shading. Use a 4B pencil for the dark shading. Use a 2H pencil to add in some light texture. Use a kneadable eraser to piick out some lights spots in the eye here and there.

Finish up your eye by shading in the eye lids using an H or HB pencil and blending in the background.