The battle map editor is quite clunky to use, there's a thread in here somewhere (perhaps in the Repository actually) that details on the map making.

Anyway, to get there click single-player from the main menu, then click "map editor". It should be the last one (at the bottom) of the list.

Here you can edit an existing map, or make a new one. I highly do not recommend editing an existing one.

Anyway once you've selected the size of the map and the terrain (desert, arid etc.) a little message should pop up saying whatever you do here is not supported by CA. I doubt it would be even if that didn't pop up (being such an outdated game). Ignore that like anybody else would.

Now, there's an internal helper that should well.... help you. Just hit the F1 key on your keyboard and you'll get all the info you need. It's (as I've said) clunky to use at first, but with time you'll get better.
If you have any troubles or questions just ask, or try finding that thread I've mentioned - it might either be in this forum or the Repository, I can't remember too well.

Good luck. Hope that helps.