Okey doke.
1. Go out into your garden and find some good clean soil. The less clay the better. Sift it into a bowl.
2. Microwave the soil for a couple of minutes to kill any bacteria.
3. Make sure it's in a bowl that doesn't matter much. Pur in water until it's a sort of thick mud, and add some thick PVA/white/elmers glue. Stir until you have a lumpy mixture that you can spread.
4. Using whatever, spread the 'mud' over you base or whatever. I did it straight onto 1/4" hardboard, so it didn't warp.
5. As soon as you have spread it to your satisfaction, sprinkle the top with more soil until it is completely covered. It will stick.
6. Shake off any excess soil. If you want tracks in it, now is the time to make them.
7. It will take a couple of days to dry solid, depending on how much water and glue you used. Check it every few hours, and if cracks begin to appear, push them wish you finger to fill them.
8. A while later, your done! Enjoy. That's how I did mine.
Pros:
Very very cheap. Everything needed is free, except for a bit of glue.
Reasonably quick and easy to do.
Looks real because it IS real.
Surprisingly durable considering what it is made of.
Cons:
Not fantastic quality.
Smells a little bit, but you have to be pretty close.
May crack.