I think you just want to start by going to move the relevant stuff around in descr_strat and give the Julii however much of Italy you want to give them - then you can start modifying the other factions into what you want them to be, give them their new provinces, etc.
List of things that you'll need to look at, from memory:
- Banners, this is fairly simple, this is just a .txt file and making/editing a couple of graphics files
- New faction icons for your new factions, the biggest type is in the loading screen folder then there are two smaller types in menus (Start by making the big ones then shrink them or they go blurry)
- UI files in the data/UI/Generic folder, two of these (can't remember which) have the faction icons in as well.
- Obviously reskinned/new/edited units
- Export_descr_buildings so you can build said units
- expanded_bi.txt in the text folder, this contains all the faction names
- descr_strat.txt to change who owns what on the map
My advice is to start on banners and units (and maybe expanded_bi), that means you can get some nice shots of new units etc done with pretty new banners and get people interested in the mod more