First, you have to find a skull ... if not, modeling is the solution !
Step 1: The Latex
You have now the skull, you need to prepare the latex.
Sometime liquid, sometime creamy ... focus thereof. Latex is used with brush directly on the skull.
If you want to save your brush, soak it before in soap.
Step 2: Application
Latex is really easy to use on plaster.
The plaster absorb water of latex, so it makes the cast fast.
On this size of skull, 0,5 mm of thickness is enough.
You can pass the latex on different layers.
Step 3: Latex Dry ...
The latex dry, you have to see if there is no against undercut.
If there is closed it !
Step 4: Use Plaster ...
Plaster maintain the latex, so it's important to make the step and englobing the latex.
When plaster is dried ... time for unmold and casting !