It's not like I really have it planned out when I start working on something. Typically I add a new layer and then start dropping different textures onto it and play with the settings. That's the best way to learn in my opinion!
That black texture either came with Gimp or was one I made, I can't remember. It might have been a red version of it that came with Gimp and I threw black onto it to change the color and then saved it as its own texture.