Pretty simple
Get a 1' x1' VCT tile from home improvement store. I got mine at Home Depot. They should sell singles
Draw a jagged outline of how big you want the crater base to be.
Score outline with utility knife. The VCT is going to be too tough to cut through.
Use pliers to snap off tile around the scored outline. Outline can't be to detailed as the tile will not snap exactly, but it does not need to be exact, it is supposed to be a crater.
Take the broken pieces you snapped off and glue them around the edge with PVA glue. You will probably need to snap some extra off the bigger tile. Snap and glue in random sizes and places til it looks the way you want.
Let dry for a day
Paint black. I use a thinned down craft paint so it runs into all of the low spots and covers good.
Make up some thinned down PVA glue And "paint" that on the surface, then sprinkle on some model rail road sand (I used black). Then kind of dust it off the high spots with your fingers.
Let dry for an hour or so.
Dry brush 2 layers of gray craft paint. I used 2 shades. 1 kind of medium gray and one closer to white.
Glue on metal bits, old spruce , old circuit boards , you get the idea just a bunch of junk.
Glue on some model rail road foliage.
Finally scrape off a couple of spots in the middle of the crater if you want to paint on some "molten" metal pools and put a little PVA glue over them to make them look a little bit like liquid.
Done.