EXTREME realism

What kind of skull? Human? If your sculpting skills are not such that you're able to create the type of realism you need, and since the object in question is a real object that can be found in the real world, you can just download a photogrammetry model of a skull. These are scanned meshes that contain all the minute detail of the real object. Then you can retopo and bake the details, or just use the skull as is. You will not be able to resell the object of course, but there's plenty of them out there with licensing that would allow you to render and use them in animations.

I can describe my work process.
First you need visual references, front and profile photos of the model, in addition to all possible graphic documentation.
With these images, topological drawings of both the model in general and the detailed parts are created, these drawings will be used to create the three-dimensional planes of the model, and contain all the necessary information.
On the three-dimensional plane formed by the combined front, top and profile views, you can start modeling, usually from a box, by adding subdivisions, the box adapts to the shapes defined in the plans.
The result should be a model fitted to the plans, although the difficult part is guessing or discovering what are the necessary or appropriate cuts and lines in each step.
If everything has gone well we will have a tight and simple model, with no further subdivisions than the essential ones, but as detailed as necessary.
And this is the point, if there is enough detail, good lighting is enough, you can even do without color and materials, and get a hyper-realistic image.

Can you perhaps post a few pictures of what your model looks like now during your best attempts?

there are many factors that need to be accounted for to make a model look realistic, and without knowing where you stand now, it is very hard to say what you need to change or improve.

also, in 3d modeling, experience is extremely important.
not saying that it can't be done, but achieving "EXTREME" realism on your first and only model is not exactly something that everybody can do, not even with the best instructions.
you might be an exception to the rule though, but please keep that in mind.