Well you can't exactly create a faction as RTW game is already at limit of 21 factions. However you can re-use and adapt an existing one - I don't think there is a specific tutorial that covers everything you need to look up each element separately.

Scriptorium or the links section should help
https://forums.totalwar.org/vb/local_links.php?catid=70

you'll need to know:
how to change faction names (nice easy bit that)
how to add new units (probably)
how to change banners and icons

probably more it depends what faction you are trying to turn into what.