So... At first there was a quick sketch. It look's like absolutely not Castiel. But that's is because there're no shadows, and lights, that make the shape of his face. Anyway here the main proportions are given. I had a 3,5 seasons of references, so to find them out wasn't difficult. :)
Making a skin texture it in another words is drawing a patches of light on the skin. The main trick is to guess where to draw these patches. To do it well you have to look after the skin in different illumination.
Then goes the famous trench coat and some light on the hair. Usually I use very small size of brush to draw hair, but here I did it in another way. Men's hair are more tough so I did it less detailed than I always do. So... that's it. Hope it was useful for you! :)))