In QuickMask, make some marks with a 100% brush. Hard Round will do. Then, blur it with motion blur on a high setting. Exit quick mask, and you should have something that looks like the last image. I think I mention all this in the next part of the tutorial. The link's above this post.
Oh my gosh. I freaking love you. About what you've said in this tutorial, I can do perspective a LOT better than I can handle light. I am really color-weak and absolutely suck at light effects. If you see my gallery, it's all black-white line-art. I've always been amazed by people who can make beautiful dancing lights, and grain effects, like the thousands of trees in a forest, without actually drawing each tree. I suppose I just need a lot more practice.
Yes! Please go on and share us the rest! I'm seriously thinking about 3D sketching. You just proved it's a powerful tool. The purists may evolve someday, if evolution was not ruling then we would still be painting our cave walls.