Nice drawing and thank you for your comments. You asked how I get a layer to gradually sink into the back. Well after I finish working on the layer, I copy that layer to the foreground so now there are two layers with identical images. I then select the eraser and drag the opacity meter of that eraser all the way down. Then I carefully erase the top layer slowly. The more I erase, the deeper the top layer 'sinks' into the bottom layer. I know it's a confusing explanation but you'll understand once you try it. Good luck.