The relative size of length and width is different. The size of the front of head vs the side of the head is also different. There are 2 parts to the skull.  The eyes are positioned on each side of the top of the nose. The nose is centered in the middle of the face. The mouth is positioned and centered two fingers above the chin? The top of the ears are lined up the eyebrows and the bottom of ears are lined up with the bottom of your nose. The neck is placed  at the bottom of the jaw. The shape of an average eye is round on the top and flat on the bottom. To make an eye look glassy i add white to where the light hits.

In this landscape picture i drew, i started with the lighter colors first and i noticed that by doing this i could use darker colors on top if i needed to fix any mistakes easily. I also made sure my colors were very bright and vibrant so it looks more presentable and realistic.