I am the first to admit I'm not a real artist.
I can't look at a landscape and paint it or sketch a friend's face and have it be recognisable. But to get respectable results for miniatures that's not really essential.
I'm not great at faces, but they are important so I've practised as much as possible and have picked up some techniques and tricks to make it easier on me and I thought I'd share them with you. Bear in mind this is not a guide for the experienced painter but for us happy amateurs.
Step 1: Base Coat
Base coat the face with a lightly tanned flesh tone, Citadel Eldar Flesh is a good base for most models. Be sure to keep the paint fluid or it will clump up and obscure the detail. Wait for it to dry before you do anything else or the colours will bleed together. The exception is if you want to blend but that's a whole other step by step guide and not really a beginners technique.
Step 2: Wash
Then once its dry wash with a watered down brown wash (not Flesh wash, its too yellow for my tastes). Your wash should run into the deeper parts of the face and help show up the details. If your wash has too much water those details won't show up, if it has too much paint it will just cover them anyway. Its better to err on the side of caution and if you go wrong just load up your brush with water, add it to the face and use a dry brush to soak it all up.
Again once its dry pick out the highlights with a lighter flesh tone, the forehead, the blade (the bit that get sunburned!) of the nose, the top lip between the nose and lip, the ball of the chin and the cheekbones. Allow the highlights to dry or if your particularly ambitious try blending the contour or low light colour with the highlight while its still wet. You can see on the photo to the left the highlights and low lights are shown.
Step 4: Detailing
The last step is to add in those tricky bits like eyes and lips. You can add an even lighter highlight if you wish to the ball of the chin, cheekbones and forehead too.
Lips are generally pinker than you think, just add a tiny bit of blood red to the Eldar Flesh you used earlier and mix it together. Paint a thin line over the bottom lip only, top lips very rarely show on miniatures except large models and some females.
The best way I've found to do eyes is to paint the eye socket with black, layer a strip of off white on top leaving a little of the black showing around the edge. Finally dot a pupil in black and be careful to get them lined up together or your model will look cross eyed!
The last detail is makeup for females, a light red wash over cheeks for rouge, the same for eyeshadow, just choose a colour that complements the model's paint scheme (earthy colours look very good for this as does blue or green). To paint lipstick I draw a Scab Red line over the bottom lip, highlight with Blood Red and finally dot a little white into the middle which makes it look shiny.
I hope this guide has helped you, if you would like any clarifications or more details please leave a comment.