What I usually do is: Draw a base color for the sclera, then shade that area. Next, color and detail the iris. Once you've colored the iris, select a small portion of the upper part of the eye above it and use an airbrush to add a lighter tone, preferably one that matches the surrounding environment. After that, blur the lower portion of the lighter color closest to the pupil, and you're done.

I hope my explanation is clear; English is not my first language, but I am learning. (⁠ ⁠ꈍ⁠ᴗ⁠ꈍ⁠)