Attention, huge file (~3.5mb)

Since I made some screenshots during painting my last deviation, I decided to make a tutorial out of this. I don't want to be megalomaniac, but when I started to draw this portrait, I nearly gave up and thought it would never be something that looked more or less good, not even to mention being finished someday.
But I kept on drawing (not at last thanks to the support of my friend ) and was quite happy with the result. So please take this as an inspiration next time you think you can't draw for your life. I hope the description on how I do things come in helpful for you, too.

The result can be found here: [link]

Since I'm not english, I bet there are some mistakes in it, but I hope everything is understandable.

Please tell me what you think of it and if you think I could change something to make it more helpful.

EDIT: What I forgot to mention: The straight-line-rule depends on the orientation of the face. In the picture I used, the head is nearly frontally seen. That means the line you would draw touching the chin runs parallel to the one in the eyes, which is here, incidentally parallel to the border of the picture. If the head is turned, the line that connects the eyes might be in a different angle, but it still has to be parallel to the line of the chin (and mouth in the relaxed position). However, more individual faces might vary, but it is a good orientation. Same goes for monsters (the more symmetrical ones, though), horns, ears, eyes should be on more or less parallel lines.