Rule a line across you're sheet of polystyrene, about 2cm from the edge. Mark vertical lines off every five or so centimeters. When this is done, divide each rectangle into two triangles, and cut them all out.
Place your wedges of polystyrene in a ring, so you know how big the crater will be. Measure it, and then use this measurement to cut a base out of the cardboard. My bases are circular, and the same size as the craters. However, you might want to make them larger, so you can fit some other things on, such as trees, ruins, or other craters. Depending on how large you want the crater, a larger base is required. My crater with corpse has a double thickness base to make it higher, while the other has a single thickness base.
Glue the wedges to your base using PVA (Remember what Cynoacrylate, AKA super glue does?) Leave them to dry.
Mix up some stiff plaster, and plaster over the entire crater. Use the plaster to fill up the gaps between the wedges. Smooth the plaster off, and leave it to dry, preferably over night.
Sand the plaster (with sandpaper, not sand) so you get a reasonably smooth finish. You don't want big mounds of plaster sticking up at odd angles.
Paint PVA over the crater, about a half at a time, and cover it with sand. Repeat the process till the whole crater, including the inside is covered. Leave this to dry, for around 6 hours.
Paint the crater all over in black. You can use a spray can now, all that plaster and sand will seal the Polystyrene and prevent the dreaded effects of spray paint. When it is dry, drybrush it with Gray. This gives an ash effect. For a dirt effect, I would recommend dry brushing with a deep brown.
You can either leave the crater as it is, or finish it off in the Pudding Wrestler fashion.

Most craters in real life have muddy bottoms. Just watch any war film (Preferably the famous 'All quiet on the Western Front', which inspired my corpse) and you will get the idea. I decided to simulate it. Paint the floor of the crater in watered down PVA, mixed with a little chestnut wash. This gives a semi-reflective muddy look. An alternate method is the famous 'AOL crater' system, named after the AOL CD-ROM which where usually used. In this method, a CD is used to form the base. It's usually an AOL CD because AOL send them out all the time, since they're desperate for gullible prats to sign up to their shit service.

Unfortunately, when it came time to build some craters for the Desert table, I'd lost my AOL CD. Fortunately, I had one from Telstra, and a couple of CD-R's I'd used for some Uni work and didn't need any more. These craters are exactly like the others, except they have CD bases and no mud at the bottoms (Deserts are not known for their mud) I have since sold all but the desert craters. They went to a good home.