I'm not someone that normally gets into modding but from the main EU site you can follow links to the improved Grand Campaign mod site (currently about to release v1.07) or get lots of help on making mods from the FAQ forum. The mods can be done just by editing game or save files. Virtually everything is in readable text format and you don't need psychic abilities or a computing degree to understand what the settings mean. Simple changes like giving a province a fort or changing its ownership, religion or population are a doddle.