1) Eyes and most body parts follow geometrical patterns. Work out what these are and draw them using the shape tools. You can then use them as a guideline to draw over. Sketch the shape of the eye roughly between these shapes on a separate layer titled 'Sketch layer'.
2) 'Hide' the geometrical shapes, and work from your Sketch layer on a new layer called 'Outline'. Fill in details with more confidence like heavy lines, eyelashes and the iris.
3) 'Hide' the sketch layer and create a new layer underneath the outline called 'shadows'. Playing around with the opacity of the brush and different layerings of shadow, fill in the shaded areas. Remember that the eyeball is a spherical shape and shade to give it depth.
4) For a colourful pop, create another layer beneath the shadow and outline layers for colour and, using a fine brush, fill in the details. Generally the colour is darker towards the centre and outer circles, with beautiful patterns in the middle band. Play about with colour layers and remember it's really easy to undo or erase mistakes.