Pawel gives good advice but you need something to which to attach the styrofoam. Styrofaom is just not strong enouogh in the long run, IMHO. For something this small, get a 5X7 inch decoupage plaque at an arts and crafts store like Michael's or AC Moore. That should be about a dollar and will have a nice finished edge. Stain the wood with any color you choose. When this is dry, drill a bunch of holes in the base then tape the edges with masking tape. Get some styrofoam. If you don't have any around the house from an electronics purchase, you can get a block pretty cheaply when you buy your base. Cut it to the height and width you need for the raised section. Apply the putty as pawwel noted or use Celluclay and cover the styrofoam and the base to give it a nice organic feel, i.e. not too smooth...this isn't a paved parkimg lot or a green at the golf course. I wouldn't worry too much about footprints unless you're depicting partially dried mud or snow. To attach your figures and make it easier to paint them, drill a hole up one leg and superglue in a piece of brass or aluminum rod. You will also drill holes in the base to correspond to the rods so the figures won't fall over.
Here's a link to a base tutorial I posted a couple years ago.