I like to make mine in a way that is easy, inexpensive and very quick. First off, you will want some base for your miniatures, so they don't fall over on the table. I recommend using bases from Games Workshop or Mantic because both have great options already prepped at affordable prices. Make sure you prime it before painting! You can use black gesso or paint white primer directly on top of the green plastic. That's fine too! Now take some watered-down acrylic craft paints (they're cheaper) and do two coats until it looks how you'd like especially if working with dark colors such as blue-gray, browns, etc. Let dry between each coat for an hour minimum. The last step is to paint on some crackle medium. This can be bought at most art stores and comes in a tube-like toothpaste (or mayo). It's easy to use; squeeze out about the size of a quarter onto your paper plate or something else that you don't care about because it will get messy. Then take an old brush with stiff bristles and dip it into the water until no more bubbles are forming (blow on the tip of the bristles if necessary) and then dip the wet end directly into your craft paint which we prepped earlier. Mix, so they blend well but not too runny, as seen below. And now you have yourself a cracked earth base!