So, this turned in to a bit of a beast, I hope it isn't too much text!
As promised, a tutorial showing how i painted this picture [link]
It requires some basic knowledge of Photoshop but nothing too complicated.
It has a lot of theory that i've picked up over the years, so it doesn't just apply to this painting, but to any painting really.
I hope it's helpful, and feel free to note me if you have any questions about anything