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I copied this over from CFC after someone requested help with adding new civs, I figured it would be good to have it here as well for those who might want to add Canada or the Zulus. 
The easiest way to do it is to simply modify two files: CIV4LeaderHeadInfos and CIV4CivilizationInfos.
The way Civ 4 currently works (and is the best way to do things) is to use TXT_KEYs for all text entries (names, civilopedia entries, strategy entries for units, etc.). You provide a TXT_KEY for these entries and in the Text XML you have associated text for each TXT_KEY in each language. The game determines which text to use based upon the active language - if you're just going to modify things for yourself and don't need to worry about other things you can just enter the text directly. So instead of "TXT_KEY_LEADER_WASHINGTON" you would have "George Washington". If you plan on having translations or use Python it's better to use TXT_KEYs.
In LeaderHeadInfos you copy one of the other leaders and place it at the bottom of the file (I usually choose a leader most similar to whoever I'm trying to make so you don't have to change as much). Each leader entry is rather long and includes things like diplomacy sounds and AI attitudes but there's only a few things that actually need to change. In the new entry you want to change the Civ tag ("LEADER_WASHINGTON" to "LEADER_SOME_GUY", for example) as well as the Description and Civilopedia text entries.
In CivilizationInfos you want to do the same thing - copy one of the other ones and place the new entry at the bottom. This file has more that needs to be changed. You need to change the Type tag like with the Leader (e.g. "CIVILIZATION_AMERICA" to "CIVILIZATION_CANADA") Description is the "official" name for a civ (United States of America) while the ShortDesc is the colloquial version (America). Adjective also needs to be changed and Civilopedia also if you care about that. You can also change the color and other stuff but it's not technically necessary.
The cities entry for civs also use TXT_KEYs but again you can simply add the names directly if you wish. Change the Unique Unit below that if you wish, the starting techs further down. Close to the bottom is another necessary entry - you need to change the available leaders for the civ you copied to the leader you've already made. So change it from LEADER_WASHINGTON to LEADER_SOME_GUY (or whatever you named the leader in the other thread).
That should be it. If anyone has any questions let me know.
The easiest way to do it is to simply modify two files: CIV4LeaderHeadInfos and CIV4CivilizationInfos.
The way Civ 4 currently works (and is the best way to do things) is to use TXT_KEYs for all text entries (names, civilopedia entries, strategy entries for units, etc.). You provide a TXT_KEY for these entries and in the Text XML you have associated text for each TXT_KEY in each language. The game determines which text to use based upon the active language - if you're just going to modify things for yourself and don't need to worry about other things you can just enter the text directly. So instead of "TXT_KEY_LEADER_WASHINGTON" you would have "George Washington". If you plan on having translations or use Python it's better to use TXT_KEYs.
In LeaderHeadInfos you copy one of the other leaders and place it at the bottom of the file (I usually choose a leader most similar to whoever I'm trying to make so you don't have to change as much). Each leader entry is rather long and includes things like diplomacy sounds and AI attitudes but there's only a few things that actually need to change. In the new entry you want to change the Civ tag ("LEADER_WASHINGTON" to "LEADER_SOME_GUY", for example) as well as the Description and Civilopedia text entries.
In CivilizationInfos you want to do the same thing - copy one of the other ones and place the new entry at the bottom. This file has more that needs to be changed. You need to change the Type tag like with the Leader (e.g. "CIVILIZATION_AMERICA" to "CIVILIZATION_CANADA") Description is the "official" name for a civ (United States of America) while the ShortDesc is the colloquial version (America). Adjective also needs to be changed and Civilopedia also if you care about that. You can also change the color and other stuff but it's not technically necessary.
The cities entry for civs also use TXT_KEYs but again you can simply add the names directly if you wish. Change the Unique Unit below that if you wish, the starting techs further down. Close to the bottom is another necessary entry - you need to change the available leaders for the civ you copied to the leader you've already made. So change it from LEADER_WASHINGTON to LEADER_SOME_GUY (or whatever you named the leader in the other thread).
That should be it. If anyone has any questions let me know.