Well then, Ladies & Gents, finally, here’s how I paint faces….
Now, please bear in mind, this method works for me. You might not get along with it but, hopefully, there might be something you can take from it and adapt.
All this is done with Vallejo’s. Here’s what I use.
Start with a base coat of Brown Sand.
Next, very lightly, drybrush downwards with a touch of Sunny Skin Tone. This will give you an idea of where the highlights and shading will go. This should be very subtle so, don’t overdo it.
Next up, shading. Add a tiny amount of Burnt Red to the Brown Sand. Remember, you only want to darken it by a shade.
This much is enough. I also make use of Flow Improver as well. Does exactly what it says! Add a few drops of distilled water. What you need to do is to build up the effect with multiple glazes.
First shading.
And the next, with a tiny bit more Burnt Red. For subsequent coats, keep adding more. Remember to thin as you add more paint.
See the effect getting stronger? Subtlety and patience are key here. At this point, block in the eyeball with very dark brown.
Now for the highlights. Once again, Brown Sand but this time, tiny amounts of Basic Skintone. Same as the shading, build up using progressively lighter glazes.
Ok, five o’clock shadow. This time, an even thinner glaze of London Grey. Build up in multiple layers.
Now, to blend it all together, a couple of glazes of Medium Skintone. These should be almost transparent.
The eyes can now be painted. Block in with Offwhite, leaving a very thin line of the dark brown around the outside to outline the eye. Dot in the iris and pupil, then apply a very thin line of Rose Brown to the lower eyelid. The inside of the mouth is painted in Burnt Red and a touch of Rose Brown applied to the lower lip.
And here we go! Finished 
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Hope this helps someone. Any questions, feel free to ask.
G.