I created a a disc at 50% gray and then drew a circle, feathered the edges (bluring it after words would work too) and filled it with white, then drew a smaller circle inside the the white one with a feathered edge and filled it with 50% grey (which makes the bottom the same hight as the rest of the disc, could be filled with a darker grey or black which would make its lower than everything else) which makes it look like a donut (kinda). I then took the hightmap into wilbure which eroded the edges to create the water channels/valleys. Brought it back into Gimp and went to filter>distorts>bumpmap which created the shaded version. Set the layer to Overlay (Or multiply for a dark look). That was ontop of a layer of the tundra texture. I then started to paint the green textures into the craters. I wont be able to a remake as I just got a commission, volunteered for my daughters field trip tommorow, and a vancouver hockey game tonight. But the obove instructions should get you started. Good luck, and post your results!