Okay...Go to your Civ III directory. You should see a file named, "civ3mod". I would back up this file, although I think there is a way that the defaults can be restored if anything does go wrong. Open that file. On the toobar make sure that "Use Default Rules" under "Tools" in unchecked. Now you can begin to edit (Rules > Edit Rules)...civs, governments, units, improvements etc.. Pick a civilization that you want to replace, rename it, add cities, leaders, starting tech...you'll work the rest out for yourself.
Unless you're good at graphics, I wouldn't bother changing the leaderhead animations...it's not worth the effort.
Currently, I'm working on an Irish civ and hope to post it before the weekend. Let me know what you come up with.