Just trial and error Maybe it helped that I had been thinking about how to generate procedural textures for a while and had started to write some simple image combining filters to try out combinations. Which is exactly what Neo does but with a visual editor.
I think it’s pretty fun to mix and match stuff using blending and so on. I usually start with a black & white texture, then blend it with something setting color. Then I use the B&W base to create a bump mapping, maybe start with the B&W base and use it to create a specular mapping. Sometimes it ends up looking good in the 3D world … and sometimes not so much Takes a bit of time and trial and error.