Whatever happened to people googling "Rome Total War mods" or something like that.
Anyways; Total War Center is pretty good, with a lot of really good mods having their own forum section.
http://www.twcenter.net/
http://www.twcenter.net/forums/index.php
You download them, install them like you would install a game (make sure you pay attentino to the instructions, sometimes you have to change the file path).
Some mods might require a seperate installation of the game (I have 3, one of which is a back up) (just install the game again into a fold, for example for Darth Mod I made new install in a folder called RTWDarthMod).
Some like the 4th Age do not require a seperate install and can be used alongwith vanilla (or the origional unmodded version).
Some mods include new historical battles, DarthMod for Rome TW has quite a few, the Napoleonic Wars mod is only historical battles, there are probably some historical battles you can add, I don't know of any though.
What TW game(s) do you have?