Is there a step by step modding tutorial i missed somewhere ?

[SilverFaith]

I am looking for the basic behavior of the module system
Do i need to overwrite XMLs completely or does the game merge similar XMLs
How to get access to the game code / SDK etc

 

[Adonnay]

This should be your starting point:

https://forums.taleworlds.com/index.php?threads/community-modding-documentation.397351/

 

[kalarhan]

Official tools and documentation will come later (from TW), right now you can see the link above plus other tutorials/threads in the forum.