Simple enough. Started with light gray, and used a circular brush of uniform color to darken circular areas. I do this with several different sizes of brush, then use "make seamless" which is crude but effective. Then I copy the image, tile it 2x or 3x on the same image (small tiles), and make it an "overlay". Once I've done this several times to have some truly small craters, I merge all the layers, copy the image, do an edge-detection on the copy so it's lit at only the edges of the more prominent craters, and make that an "addition" filter. THIS IS A HEIGHTMAP- I used it alongside the Solid Noise layer for the terrain, and bumpmapped it for detail once I had merged the layers and fiddled with the colors.