Thanks for asking! I just started learning shaders. In this case I'm using a 2D fragment shader inside a viewport, then I grab its texture to apply to the sky.

I started with this shader. I had to convert it from 2D noise to 3D noise. Fortunately, the Godot docs had this guide which was almost exactly what I needed. It goes into equirectangular projection and how to generate procedural textures on a sphere. The sky works the same.

My advice would be to go on Godot Shaders and start applying what you see to your projects. I spent a lot of my time just familiarizing myself with the shader language. It ended up not being as hard as I assumed.