1. Make sure your foreground color is black and your background color is white.
2. Filter > Render > Clouds
3. New Layer
4. Edit > Fill (make sure the "Use" is at "50% Gray" then click okay)
5. Set the new layer's mode to Hard Mix
6. Alter the rendered clouds layer using the brushes
7. To get the "baseline" layer, merge them together.
So there you go, two layers, alter the bottom one. The 50% Gray is the one ABOVE the clouds layer then merge them when you are done.