There is a long list of guides to modding Total War, focused on TWWH, by Cryswar, covering nearly all aspects. However, they're all still in Total War Warhammer II, presumably because they mostly haven't changed and so porting them over would be tedious, but they have changed slightly, so they'd need to be checked for changes, which would be even more tedious. However, they're still the best resources for TWWH3.


So, unless and until the man himself makes a TWWH3 guide or three, here's the big list, ordered in approximately ascending order of complexity/prerequisites. Links to all can be found at https://steamhost.cn/steamcommunity_com/id/Cryswar/myworkshopfiles/?section=guides&appid=594570&browsesort=myfiles&browsefilter=myfiles&p=1


Modding Frequently Asked Questions
This guide is primarily aimed at newer modders who aren't sure where to start making changes they have envisioned, but may be helpful for focused modders looking to branch into new skills. It will not go into extreme detail on answers, but will offer overviews and link to the relevant guide when possible for more information.


Modding Etiquette & Guidelines
Rather than a guide on learning how to mod, this is more about learning how to mod /well/, covering naming conventions, compatibility-minded design, and some common pitfalls.


Creating Custom Units
This guide is meant to walk prospective modders through creating a new unit in TWW2 using RPFM, assuming little to no knowledge of modding.


Creating a Custom Legendary Lord
This walkthrough will walk you through creating a bare-bones custom lord and adding them to the game. It assumes you know the basics of modding and how to create a new unit, covered in the previous guide.


Creating and Editing Abilities
We will cover how abilities of all types work - unit abilities, augments, army abilities, vortexes, etc.


Creating and Editing Character Skills
Here we discuss how to edit or create custom character skills.


Ancillaries (Items, Mounts, etc)
This guide covers the creation and editing of ancillaries in detail. After you finish, you should be able to make your own unique weapons, mounts, banners, followers, etc.


Missions and Dilemmas
This guide covers the creation of dilemmas and missions, also known as quests ingame, and many of their variants, as well as the scripting potentially needed to activate some missions.


Effects
This guide explains how effects work in TWW2. After reading this, you should be able to make your own custom effects and link them to the desired bonus values.


Creating and Editing Buildings
This guide will muck about with buildings, explaining how to edit/create them and offering ideas of how to make unique and interesting ones.


Creating a New Faction
This modding guide will walk you through the basics of creating a new faction using an existing military group (ex. Empire, Chaos) and touch on both horde and 'normal' factions.