The GL shading language itself is not that hard, you can learn most of what you need by reading this and doing the exercises directly in the web page: https://thebookofshaders.com
The difficult part for me was understanding how it connects to Monkey2 and Mojo, and it took a lot of trial an error. That’s why I put together those examples, so that people trying the same would find it easier.
However, those are all 2D. I still haven’t successfully made a 3D material with its own shader, although I haven’t tried recently.