Form and Shape
Let's talk a bit about form and shape, and what those mean when you're drawing. While not strictly 100% on topic to this thread, it's quite important, and something which you do need to know. Also, I couldn't be arsed to create another thread.
We'll start off with shape. That's the two-dimensional image. For instance, a fox is vaguely dog-shaped, even though the two animals are quite different. If you were to see the outline of a fox, you would be forgiven for thinking that it was something else. Similarly, if you see the outline of a fox, you still have a fairly good idea of what it could be. Depending on how much of the outline you can see, that idea will become even more solid.
So, let's take a look at this. Even though he is quite stylised, you should have, within a certain margin of error, a fairly good idea of what you're seeing:
It's just an outline, but there are certain elements which should stand out to your brain as being specific to a very specific group of animals.
Did you answer 'rat?' Good. I'd have also accepted mouse and (given how Disney and Warner Bros like to draw them) weasel.
Now, what about these drawings suggest that it is a rat, and not something else? The ears are probably the biggest give away. They're large, round, and thin, as opposed to pointy and thick. His muzzle is another part that tells us what sort of species he is. Unlike a canine's muzzle, which is more blockish, his is more conical, coming to a point at the nose.
So, why don't we break all of him down into his simple shapes. In doing this, we'll get our form.
Again, his muzzle is sort of, but not quite, conical. His ears, in this case, are sort of cylindrical, as is his neck. It's his face where things get really tricky. The bridge of his nose is set further forward than his eyes, which are back in his skull a bit. The top of his head is rounded, but the sides of his face are sort of flat. Even though he has fuzzy cheeks, they are NOT round. Even if he had cheeks of a rounder shape, they still would not be spherical. They'd be slightly convex, yes, at the result of his little triangular cheekbones simply extending further back toward his ears a bit, which, depending on the light source, might let the light fall on his cheeks more than it would if you had a more human-shaped face.
Speaking of light sources:
With the light on his upper right (our upper left), all of the shadow wants to go down and to his left. His muzzle casts a bit of shadow on his neck, and his ears are shaded on the bottom, and lit on the top. His left shoulder is in shadow, and his right is not.
How this same light source would look on top of muscle and cartilage and bone of similar shapes would be like this:
Everything's more or less the same, but slightly more detailed. There are more ridges and valleys on the actual character than there are on the simple shapes, but even those extra details can still be broken down to the same sorts of simple shapes.
Now, I have left the hair unshaded, because while it follows the same rules, it is a bit more tricky.
Hair is not one solid mass. It's a bunch of little tiny bits that come together and will look very solid if you shade it incorrectly. What you have to do is define those little tiny bits. Obviously, you don't have to define EVERY ONE of them (nor should you), but you do want to give the impression of bits and pieces that don't all start and stop in the same place. You can even do this with cell-shading. You'll probably want to spend more than three minutes on it, but even with just that, it's clear that there's more going on than just an oddly-shaped plastic helmet.