Eyes

First, You need to know about real eyes, it is based of some parts of real eyes, some things are really like real eyes.
See the sketch below:


We have a pupil, a reflection, eyebrows, eyelid and eyelashes, all parts of real eyes.

This is a normal and pretty common manga eye.



alright, You need to get the shape of the eye, You can use different kind of shapes.


the pupil is done after the shape, You have to put the reflections of the light with it. Defines the white part os the eye, that is all free to make over the pupil.

The Eyelid is all optional to give it a different look, it is done between the eye and eyebrows.



and the eyelashes, can give it a girly look, You can put as you wish, judt doing the lashes on the contour of the eye, Look at the picture:



And you are done with the eyes.

Nose and Mouth
first look at real nose and mouth:

The nose and mouth is a little different than real ones,
they are less detailed.
The nose is just made at one of the sides of a real nose, quite simple.
They are all alike.
About the mouth, at the empty expression it is just two lines, one of the middle of the mouth, marking upper lip, and one shorter at the bottom, marking lower lip.
Expression works with the mouth together with eyes, the combination of both makes a definitive expression.
Look at some examples of nose and mouth:

Ears
The ears have no secret, they are just like real ears.
Look:

Now the manga ones:

Expressions
Like in real life,
manga characters has expressions showing his feelings about something.
To texteures the expressions will just change some parts of the face.

Normally the eyes, eyebrows and mouth will change.
You can change the face to express anger, or sadness as example.
Look at this expressions below:

happyness, sadness and anger,
You can express emotion of your characters in faces as you wish.

Hair
the hair, like a real hair, it is composed of strands, but it is made of a few strands, not each one.
A real hair looks like this:

Strand by strand for a real hair.
But with a few you have this as anime hair strand:

like a few strands of real hair.
If you looks close, You can realize that hair is more curvy at the bottom of the hair strand, I heard somewhere it gives it more anime look.
The hair is as detailed as you like.
You can give it flow as you wish, all the hair must follow the direction of the flow, like it is moving.

Mapping
A good texture, often needs a good mapping.
Here I talk about the mapping of head textures.

First, proportions.
No matter what you draw, You must follow correct proportions.
Like in reality or any manga face or body drawing, you must have the correct proportions, else it will be a junk.

Human realistic drawings and manga are alike,
You must take the distance you are drawing the facial features first.

lets start with the eyes.
On the eyes, you are free on the size, just don't mess with the general face size.
In the placement,
it is one eye in between the eyes, almost one eye to the ear, take a good look where the eye is going to be.

now with the rest of the face, it is one nose, under the eyes, not too distant from the eye, but might have a good distance sometimes,
the mouth is right below the nose, just a little space under.
About the size, the nose is free size, just not too big.
The mouth is from one of the eye to the other, no matter where in the eye,
Just in the middle of them.

Going to the sides, the ears comes on the seide of the eyes, can be half eye of distance or a bit more.
The proportions of the ear is like real human ears.
From the most top of the eye to the end of the nose.

The jawline is optional,
You can place it from the ear, passing under the mouth close to the last circle in the texture template.
it can be a good option to make the head more mature.

The hair could go above the eyes, letting an empty space for the forehead.

Some types of general mapping:

the first is a low face, it can work sometimes, but makes the whole head so childish and big looking.
the second is normal mapping with the chin, totally correct, I cant see anything wrong.
the third is just a small face and how it maps, it si a big mistake, just like a too big face, maps really bad.
And the other the notmal mapping without the chin.

This explain a lot about mapping differences.
Make your own mapping, and make sure to make it look good and proportional.

Coloring
To color your head,
select a color of your choice for skin and for the hair, pupil and what else.
paint with the plain color erasing out of the outlines.
If you want a good head, you need to shade, work with shading on the whole head, the color is of your wish, just see where the light comes from and draw the shadows.

Under the chin, under eyebrows, under the nose, under the mouth, edges of the hair, the ears, where the shadows goes are mostly this areas.
And so the light where the light hits, mostly on hair, eyes and a little on the nose.

To use gradient of colors in the skin or the hair and eyes, gives the head a good look, don't avoid it.

Now that you know the basics, try it yourself!

This is a video of an anime head texture.