First i spray prime paint whit grey color.
Then i paint shadow and extraordinary places whit pure black .
Next is painting little white extraordinary places.
here and there, thin lines.
Last i dry-brushing all whit white.
Then use earthshade all over rocks
Then i make very light green and pressing here and there almost dry brush
Paint whit water and pigments some rust strikes.
Finally its looks likes rocks...or i hope so ;)
I hope its help you to rocks painting.