"But I don't want to learn formulas! I want to learn to draw."
Drawing with a marker requires you to make a lot of very fast decisions. If you're new to this, you can be overwhelmed and disheartened very quickly. This stuff is hard to learn.
Learning formulas to quickly draw features will remove one task from your crowded brain. You'll be more able to focus on placement, line weight, shapes, planes, and carrying on a conversation.
Once learned, these formulas can be adapted to any face, changed, or forgotten. The important thing is that they make your task more endurable now.
1. Draw a thick line on top.
2. Draw a thin line on bottom.
3. The iris is a perfect circle... but don't draw the whole circle.
4. Draw a shadow on the iris, cast by the eyelashes.
5. Put a highlight in the eye. Off-center, not too big.
6. Put the pupil right in the middle of the iris.
7. Add an eyelid.
Do this until you do it without thinking. Most of my new people forget to draw the shadow (step 4) after awhile. Don't forget the shadow. It adds a lot of subtlety and depth.
How the eyelid sits over the iris. Usually, the eye opens just wide enough for the pupil to be exposed, no further.
Don't forget to position the highlight below the shadow.