Start with an outline. Draw lightly. Don't be too heavy handed at first. Get a feel for the pen on the Bristol board.
Next, give your eye a bit of dimension by adding creases at the top and bottom of the eye outline. Also, draw an outline of the iris.
Begin giving your eye some life by sketching in eyelashes.
Start getting more deliberate with your pen by making the lines of the lashes darker. Color the pupil, but leave a bit of it uncolored. This acts as a highlight on the pupil and will add contrast.
Keep sketching the lashes, making them darker. Also, do a bit of light shading in the iris. Not too dark as to not blend in with the iris.
Next step is to start adding the eyebrow. Very easy to overdo this touch, so have a steady hand.
Continue to shape your eyebrow and put the finishing touches on the sketch.
And there you have it. A realistic eye and eyebrow drawing in pen.