I am asuming you know a bit about xml modding, preferably GC3 xml modding. Download the mod from the external link above. Extract it to your Mods folder. Boot the game, no seriously, boot the game (the mod is over in 5 turns, you will see ), pick a random Leader and see what its all about.

Did you boot the game? Probably not!  

Now you have seen what the mod does. If you look at the xml's of the mod, more specificly MyMod_IdeologyChoice.xml, MyMod_Ideology_B.xml and MyMod_GameEventsDefs.xml. I have left comments in these 3 files that I hope contains all the information you need to understand the basics of the Event system and get your own event chains going! (Not overly great at the explaining lark tbh)

Includes a one off event example and a event chain example. Feel free to ask questions, post your own info on what you have gathered with your own experiments!