Hi Paul, Andrew has given you some links above and there are quite a few face painting tutorials on You tube, also worth getting hold of the Cartacci Book if you haven't already got it.
I have given up using acrylics for faces on anything above 54mm as not only is blending tricky but the flat finish I find unrealistic. I have just downloaded issue 12 of the Illustrated Historical Artist and there is an article on painting a face with acrylics and overpainting in Oils-haven't read it yet but another technique worth looking at.
Keith