I think I'll go in rounds, adding a few at a time to a super-mod of mine.
1: Fictional civ that I make up myself, just to test out adding a civ. (This is bs with no scenario editor, just a "powerful" world builder, I have to go into crappy text files to change the game with no GUI)
2: Babylon, Israel, Scandinavia, and Ottomans.
3: Poland, Celts, Iroquois, and Carthage.
4: Austria, Sumeria, Netherlands, and Hittites.
5: Portugal, Zululand, Assyria, Hungary, and Cambodia.
6: Sioux, Serbia, Olmec, Tibet, and Ghana. Then I will have created a 36-civ mod just in time for an expansion and pack and having to do it all over again.