I have worked with this stuff for years. I got my tips from a guy that used the stuff to make water falls in comercial buildings.
Spray on the basic shape you want for your feature. wait a few mins and then come back to it. dip your hands in water. Now you can test out a section by lightly pinching it. As the great stuff cures it gets harder to pinch. by pinching, poking and pushing in sections you can make ridges and gullies to simulate rock. its best to keep comming back to it as it hardens you can do different levels of sculpting. with this technique you avoid the spongy look you get when you cut this stuff and get a more natural look with a smoother surface. Next I spray paint the whole thing(use only non toxic paint). I fiirst paint the whole thing black, then use gray, brown, beige, silver and white. use these colors in small amounts spraying in angles so that the black still shows through and best to work from dark to light in colors.
as you spray the lighter colors over the darker colors working in angles allows the dark colors to show in the cracks nooks and valleys of the surfaxce giving you a natural rock like look. at the very end I use a fine artist brush to to highlight different areas with the darker colors and to touch up areas.
Hope this helps
Bob