Just a very quick summary of the process:
1) Open Civ4CivicInfos.xml. You find this in the ...//Assets/XML/GameInfo folder.
2) What I normally do if I want to add an entirely new Civic option is to Copy & Paste the one above it, and alter the names and functions accordingly.
3) If you are creating an entirely new CATEGORY of Civics, then you will next need to open Civ4CivicOptionsInfos.xml, and add the new category here. Otherwise, ignore this step.
4) Next, open up the Civ4GameTextInfos_Objects.xml file, which you will find in the ...//Assets/XML/Text folder. Find the Civic tags (e.g. <tag>TXT_KEY_CIVIC_DESPOTISM</Tag>) and add in the tags for your new civic options.
5) Again, if you have created an entirely new category of civics, then find the Civic_Category tags (eg. <Tag>TXT_KEY_CIVICOPTION_GOVERNMENT</Tag>). Again, add the new civic category tags.
6) Once finished, load up your mod and keep your fingers crossed

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Oh and, btw, if you are modding the XML files, you CANNOT go past Altova XMLSpy. IMO it is the ONLY way to edit these files in a simple, 'idiot-proof' fashion

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Oh, btw, before you alter ANY of these files, PLEASE make sure you have first copied them into an assets folder within a new folder-in the Mods folder-with an appropriate name for that mod!
Aussie_Lurker.