1. New canvas, fill with white
2. New layer, black brushes, don't overlap too much just cover the canvas. Merge down, and auto contrast.
3. New layer, fill white, brush black, auto contrast, lower opacity between 50-65%
4. Repeat step 3 until you get something you like. Then merge all layers and auto contrast it. If the auto contrast sucks, just step backwards.

*Auto contrast is always optional.

This is just one way, there are many more. Hope it helps.