Hm, well I would say this. How I do it is based on who the character is and there personality and mood swings that follow. Like if she/he is feeling sad try different look about the eyes. Then there always and pair of eyes how the look in general . But also not make the same size about it like ever time. Looking to the side,down, or up. Are a bit different. How I learned is by shapes then inside the eye colors and like an eye would look like. Sometimes I think you can just color it but also using different styles might be more helpful how you want it to look. Sorry if dose not make sense or I explained it well. I usually draw it then I know. I tried. =/