Theres a really good link to painting heads on missing links. Painting 1/35 scale heads in 8 steps by Mark Bannerman.
Step 3 - Painting the Eyes
Drop a little spec of enamel Flesh Pink to white oil on your palette and bring the mixture to an off white-pink colour. Before you do the eyes, the sockets should be dark as a result of your umber wash. Dip your toothpick/needle in your whitish mixture and place a tiny spec in the inner and outer corners of each eye socket. Basically, you've put in the whites of the eyes, leaving the middle area with a dark residue from the original wash . Let this cure for two days. Split toothpicks in half and you will note that the halve-ends actually break into extremely thin splinters. You may need to split a dozen before you get that perfect splinter. Dab a splinter into a brown/black oil paint mix and place a dot in each of the sockets between the two specs of white. I tend to avoid straight head-on glares, and generally place irises looking slightly off centre, either looking left, right, up or down. In my experience, straight on looks can never be perfect! Let the paint dry thoroughly for three days.
See here for full article;
http://www.missing-lynx.com/articles/figures/mbheads/mbheads.htm