The creator of that Amazing Fan-Made Diablo 3 Level has returned to the original art forum thread to add lots of explanations and visual tutorials for those who want to understand his method, techniques, and tools. The info is way too detailed for those of us who are not aspiring computer artists, but the images he’s put together to show the process are worth a look, and you’ll appreciate the whole piece more once you see just how much time and effort it took to create it.
The image below is just a sample cropped out of one of the huge tutorial images. Click to see Tutorial 1, Tutorial 2, or the Art Assets in large, beautiful, readable size. Or browse all the images from this fan-made level in the gallery.