Don't know which one(s) off the top of my head, but at least one of the older downloadable ones has an article on boards. Should be in the contents. There's also a guide in the Starship Troopers painting guide book. If you only need something fairly basic, at least to start with, you could use MDF as a base (either one piece or in sections) and use textured paint or Plastikote-type sprays (matt emulsion and a few handfuls of sand work well enough too) and use polystyrene or foam blocks to create hills and such (you can pick up expanded poly sheets intended for insulation dirt cheap, although it'll be necessary to seal them with watered-down PVA glue to prevent them shedding chunks). Any DIY shop should keep these materials. Some companies do prebuilt stuff but these can be quite expensive for the better quality ones. Just type "wargames terrain" into google and you'll get a list.
You can see a small section of the board in the gallery (SST MI, TAC UAV bomb run) although this is in it's 'raw' state before texturing.