well i have tried making tuts, they are way to time consuming to me, but ill try to explain., you basically can start with a new image add some clouds and use some other filters till you get a grungy look go to define brush and then save it. For abstract brushes i use the brush tool put something in the center of a transparent background and use the smudge tool and move it around with different flow. Then i define brush the same way i did with the grunge.