General nation mod advice? (and specific nation idea)

[Belac]

I've never created a mod of any kind, and I'm not sure how to go about creating new units or any of the other stuff that goes into a nation. I read through the tools and tutorials link but am still confused about what I would need to know/download. Any advice would be appreciated.

My specific idea is for an Arab/Turkish/Muslim-flavor nation (call it Hajis). Any more details to flesh out the idea would be appreciated as well. What I have so far:

EA Hajis would be tribally-focused, with horse archers, elemental mages, and cultural influences from Caelum(Persia), C'tis(Egypt), and Ermor(Rome).

MA Hajis would be based on the Egyptian Mamelukes and other Crusades-era stuff, and would have good assassins.

LA Hajis would be based more on the Ottoman Empire, with Janissaries as a sacred, cap-only unit, some commanders with Sailing, and siege bonuses.