This is really a loaded question ;)

Now....how far do you want to go.

There is a good tutorial on how to change the faction name. You need to make changes in 3 text files and that does that. The units are unchanged....all it changes is the faction name:

https://forums.totalwar.org/vb/showp...59&postcount=5

Now...if you wanted to change the faction flags, that can be done too. I think there is also a tutorial for that.

Finally....there are the units themselves. If you want to change them, you can change the shapes ( need 3DS Max ) the textures ( good paint program ) or the stats ( edit the unit data in the text files )

Ultimately, you cannot add a new faction. There is a fixed limit of 21 ( 20 plus slaves ) but you can change just about EVERYTHING else!