This is a technique I learned from my father when I was about four years old. He said, son, just move the pencil. Actually, he didn't say that, but that's the principle. Just move the pencil around in random shapes. Try not to repeat the same patterns again and again and see if you find something. And then, when you finally find something, just hone in on it and start developing it. Start softly and then go harder and harder as you define the shapes. Eventually, I actually swapped my pencil out for a thicker and softer one for those harder, deeper lines. Then it's all about adding to it with the pencil, taking away with a kneaded eraser and softening with a paper tissue. And doing it again and again until you refine the look. Just remember to keep the wild lines in there because you want it rough.