Any advise would be welcome.
If you want to see how I did it, go to thislink, It may help you, First I did the planes of the skull, that helps a lot, and then you go making details.
Thank you for helping out.
Wow! that is some change! nicely done!
The only big problem is the nose hole, it is too low, normally it start at the same height of the cigomatic bone. Are you making some special ethnic skull? or just some generic one?
I was trying to do the first skull on my reference image, i thought i got the nose right, well i can pretend he was deformed somehow 
In the references of your image you can see that too. but well is not a big deal if you are not a crazy person like me hehe
nice, but maybe FYI you can find good skulls on blendswap as well.