Thanks.
No tutorials, just me, but I've heard some other guys use it, read it in one guy's post long time ago.
Basically, you take the simplest brush with soft edges, lower the opacity and flow and do it on many different layers. That's basically it. + You can change the opacity and the flow a bit when changing to another layer, so it would be different in some parts etc. I also change the size of the brush once in a while + the color just a bit, usually it's white and sometimes a bit grayish blue.