I'm assuming you may have already been googling for tutorials, but this is one I found that may be useful. It's not on a fabric doll, but the basic principles are the same.
http://www.cdhm.org/tutorials/painting- … -eyes.html
I've not ever painted eyes onto fabric, but I have painted them digitally. You basically start with the whites of the eyes, and draw them as circles. Then I elongate the circles into more pointed oval shapes at the sides. From here on in, you basically start with your dark circle (eg. dark blue), then do concentric circles within this getting lighter each time. Dragging a toothpick from the centre of your eye to the edge of the iris will create the patterns you see in real eyes. A few little flecks of lighter or darker shades here and there will also make the eyes look quite good. Then you just add your black pupil in the centre, and a couple of tiny white dots at the edge of the pupil to add some life to the eyes and they're all done!
Don't know if that helps at all - I know pictures are better and I'm really bad at giving instructions!