Rangerboy,
Have an idea what you want to do first. Although sometimes the build as you go plan works, it is better to have a "snapshot" in your mind first so that you can have a plan. This will save you time, effort and place you closer towards the result you are looking for. I usually sketch it out first and figure placement, scene dynamic and "storyline".
Demono69's suggestion is really good in that you can make a whole variety of surface contours and it sculpts easy. If you have access to a well stocked home supply store (i.e.Lowes or Home Depot) you can find the foam in in different thicknesses from 1/4" to 3" thicknesses. They carve up pretty good with a wire brush and a hacksaw blade. They also make a special glue or adhesive just for foam so you can glue it together and stack it or assemble buildings or structures. All pretty inexpensive.
Bases can be made from plywood (but attach some quarter round or trim to the edges for a nicer, more finished look) which is stable and won't warp. Hardwood, more expensive and hard to find in larger lengths. May warp split or check, especially if not sealed properly and you put things like wet celuclay on it but looks good. Or my favorite MDF, cheap, stable, machines wonderfully and can be painted to look like everything from wood, stone, plastic or metal. Comes in 1/4", 1/2" 5/8" 3/4" and 1". A 3/4" thick 4'x8' sheet will cost about $11 and you can get them to cut it to manageable sizes.
Use the foam for big contours and building up terrain, then celuclay (aval. at craft stores) for your final ground covering. Mix in some plaster and some white glue and apply to your foam or base. (This is where the "snapshot" comes in handy. If you know what you are going to cover, the easier it is to mix up rght amounts and where you are going to apply it.) While damp add your ground material, dirt, sand, pebbles or rocks. The glue will help keep it attached.
From their you can add the rest of your detail.
Good luck. If you have any other questions feel free to e-mail me.
Mike
Mike
"Imagination is the dye that colors our lives"
Marcus Aurellius
A good friend will come and bail you out of jail...but, a true friend will be sitting next to you saying, "Damn...that was fun!"