Depthy isn't that hard if you're ready to put some work into it.
Things that are far away from the viewer will be bright. I would recommend starting with the bright points and painting the whole image in that gray shade. It is usually the background (a wall or something) so it will probably fill a big area. Then you gradually darken the paint the closer one object becomes.
I uploaded my own depth map of
>>3865647 so let's take that as an example.
The wall is the farthest away, so it has the brightest gray shade. Then comes her head, her shoulders, the arm and the butt.
If you want to create round surfaces (or tube-like surfaces), the middle-most point has to be the darkest (take her ass cheeks as an example).
Of course, the penis in this picture is very close to the viewer, so it has a near-black tone to it.
Important: This depthy stuff has to come naturally in my opinion. Don't try to put too much thought into it. Everyone has a sense of depth, so it should be completely natural to know which point would be the closest to you. Also, don't go overboard with depthing. Even if you want the breasts in your image to look big, it WILL look ridiculous if they are much closer to the viewer than the rest of her body. Using well drawn images with sufficient shading is very helpful in the beginning.
