1. Print screen your avatar without eyes(drag the eye layer under the base layer).
2. Paste it in the art software you have.
3. Enlarge it to 200%
4. Set the transparency of the layer the print screen is on to 50% (or whatever you are comfortable with)
5. Make a new layer on which to make the line-art for the eyes.
6. Make a new layer, drag it under the line layer, and color in the eye white. You can make a grey line within the outer edges to mimic shade either on this or on a new layer, I prefer to do it on the same layer.
7. Hide the eyewhite layer momentarily and make a new layer on top of it. Draw the lower lid-line in pink. You can choose to do the shading on a new layer, I prefer to lock opacity and just color in the shade myself.
8. Make a new layer above that one and draw the iris. What I did was just add brown with a regular brush to make the shape, lock opacity, and used a softer brush to make the shade.
9. Now make a new layer underneath that one to very easily make a darker outer edge around your pupil.
10. Make a new layer and drag it above the iris layer for the pupil.
11. Make a layer on top of that to add a little twinkle in the eyes.
12. Make a layer on top of that to add some shadow for the eyes. A lot of BJD owners put a glossing agant on the eyelids, so I added some white sparkle on top of it.
13. Make a new layer above the line-art layer for the lower lashes. Draw them.
14. Make a new layer above that one for the upper lashes. Draw them. (make sure you zoom out by 50% from time to time while doing this to check if it's the effect you want)
15. Make a folder called eyes and move all the layers in there.
16. Zoom to 50%, this is how it will look at actual avatar size:

Save your file twice, one as original one as resized, hide the layers that are not in the eye folder. Resize your image to 50%, save as png, and voila: