Edit: I would like to say that I am in no way perfect, that I still make mistakes, and am still growing!
With a lot of manga/anime you see the mouth halfway between the nose and chin. Now, this is sort of true, but sorta not; it's actually halfway between the nosetip and chin. The more common measurement (and kinda easier) is that the mouth is 1/3rd of the way down between the bottom of the nose and chin.
Too lazy to edit the image. >w<
I was drawing with a friend on openCanvas, trying to give tips and techniques onto the measurements on heads and how things relate to each other and stuff like that.
As I was doing it, I found myself figuring out some interesting things myself.
I decided to do a head study/tutorial to try and help people draw heads better.
Please note that I am entirely unprofessional and these are just things I have learned through years of drawing, tips from others, and art classes. My techniques and such may not be entirely correct, but they are things I have noticed and am pointing out.
If you have any questions or can't read my handwriting, let me know and I will try to answer and explain anything you need.
I hope this is a little helpful.