You might look at a drawing like this and think that it's complicated because there so many lines and wrinkles. But if you understand form and hierarchy, you can easily make sense of it all.
I'll start with a simpler expression:
Once you have your basic composition and attitudes in mind and everything roughly constructed, the next most important thing is the facial expression. It should read easily, non-ambiguously and tell the story. Here George is happy and eager in anticipation of what surprise food is in his mystery can. The boys are just as eager and show a little more innocence in their expressions.
It's important to place your expression around the same construction you drew for the overall shape and position of the head. Otherwise the expression won't seem part of the head. It will just float in front of it as if it exists on a separate plane.
Then the facial features themselves (like every other part of a drawing) should have their own forms. George's eyes bug out and so they have to be carefuly drawn. Many cartoon characters (like Slab N' Ernie) have eyes that are just drawn flat against their skulls. That's OK too, if the eyes wrap around the skull shapes in perspective. Like these:I see lots of modern cartoons where the eyes exist in different planes and spatial positions than the heads they are supposed to be part of and that's wrong. It hurts to look at too. I remember when Nickelodeon did their versions of Ren and Stimpy, the good drawing flew out the window, and they just became abstract flat characters. Then that style (not mine) influenced a hundred cartoons that came after. Weird for weird sake instead of weird feelings tightly controlled by the artists.
In each of these drawings, the expressions are clear, and they wrap around the head shapes.
Next: adding details that follow the positions of the forms they help describe.