Process is quite simple : from the more far to the closest (and therefore the most detailed). I try to sketch quickly the global image and then I had more and more details (though it's important to remember that the more far it is the less details it needs). The number of layers is not constant, it depends of my mood mostly but 1 layer per ground can be useful to play on the depth of your illustration. For custom brushes, I change very often, try to find / make new all the time (last I found are from a guy called "Djahal" and are really good). And then for references, I get inspired by many things : other artists, photos, books, ideas etc... and then I make some research. Hope I answered your questions ^^.