What does it take to give a custom race a unique face?

[Kind9]

I decided to try my hand at creating a custom race for skyrim. I managed to combine some resources for the body from various mods and it looks pretty good in game. I'm not sure about how to change the face though. I want to do this right and I want it to look good. So could someone perhaps explain what needs to be done, or even link me to an in-depth guide?

Looking at the face data tab I see a list of face parts and several fields for specifying .tri files. Which of these things are essential?

 

Edit: Actually the tri files are part of the HeadPart objects.

[Kind9]

Anyone?? I see there is plenty of information about tri files out there, but now I want to know what the difference is between those 3 fields in the HeadPart object(tri, race tri, chargen tri).