This is the first of a few lessons on making custom shaders, from scratch.
Blender is adding more features to make these sorts of things easier, but I tend to like building them myself. Partly because it gives me more control over them. And partly because I don’t have to fight with some other coder’s idea of what the end result should look like.
Either way. Here is the video. (Below the video is the blend file I made during the video, for you to examine, or use, or fiddle with.
Enjoy.