alot of winging it, but generally with rocks your trying to go for a chunky jagged feel, so those random sharp edges jutting out, whats hard is getting a good directional flow through your rock so it doesnt just look like a noisy blob of chunks. Use heaps of refrence and just keep going until you start to like it. Also you dont want your rock to be chunky everywhere ideally your aiming for like 70% plain almost flat rocks with about 30% detailed sectioned spaced out nicely.