Drawing - #1 - Attempting to visualise a human skull

I'm told in order to become a good 2D artist, one has to be able to visualise and break down objects in his mind in 3D. Like the following example from Preston Blair's cartoon Animation:
So I started by breaking a head apart in my imagination to a cone like thing for the chin and a Sphere for the back part of the head:

I figured the chin didn't look nice, so in order to see what was wrong with it, I drew a side-view of the same head;

Which helped me understand what was my perception of the broken skull. And now that I knew what it was on paper, I could fix it:

Not knowing where to begin to 3D-fy it, I just extruded the surface in my imagination, Google SketchUp style;

I tried to cut the edges little by little... I tried extruding a part of the back surface and joining it with the rest of the "model"...

That thing is (for me) very hard to imagine from different angles as a beginner, So I decided to take basic parts of the skull I imagined in 2D from sideways, imagine those parts in 3D and then try to fuse them. Starting with the sphere part.
That'd probably work better. We'll see if it will or not!