There are two ways I do it.
Keep in mind that nobody really taught me, though? Like I didn’t go to school and learn the right way to do it. I just figured out a method, and it might not work for you.
The first way, I paint some colors, smudge them around to where I want them, and then slowly layer in scribbles, smudging and scribbling until I get a rough, natural look.
The second way, I paint with a soft brush and slowly add details without any scribbling. I still use the smudge tool to blend some, but I don’t push the paint around nearly as much.
The first way is a lot more like drawing with pastels, and the second way is a lot more like actual painting. But they both work pretty well for me.
Hope that helps!