This guide is based on what BlazeDragoon told me in his 4 player mod thread (link below) I just formated it as a simple guide.
First you want to read these guides written by the Larian team:
Setting up the editor:
http://steamcommunity.com/sharedfiles/filedetails/?id=276611375Setting up your first mod:
http://steamcommunity.com/sharedfiles/filedetails/?id=276641529Now you know how to start your own mod, but if you want to edit the official campaign it seems to be way more complicated:
Solution 1:
Download the 4 Player mod, and set the player limit to what you want:
http://www.larian.com/forums/ubbthreads.php?ubb=showflat&Number=504225Solution 2:
1. Start up the editor
2. Choose: Create new module... and click "ok"
3. Type in a name you like and select Main in both under Mods and in Data Packs, click "Create"
4. Now click on "New level" and enter "CharacterCreation" (of course without ""), after a short loading period you will see the Level from the Main campaign
5. Click on "File" in the menu bar and then "New Level" in the context menu, enter "CYS_BlackCove"
6. Do step 5 again with the following list of level names:
- Cyseal
- DAF_Druid_Dungeon
- DAF_Inner_Sanctum
- DAF_King_Tomb
- DarkForest
- Homestead
- LUC_Mines
- LucullaForest
- Menuscreen_Cyseal
- Menuscreen_Luculla
- ThePrisonGardenOfStoneKing
7. After "creating" all those levels click on "Modding" in the Menu Bar and then on "Module Setting" in the context menu
8. As startup level choose "CharacterCreation" and as Menu Level "Menuscreen_Cyseal", set number of players to the desired number and click save.
9. Now you can add new things to the main campaign but beware, as it seems you can't modify existing entities in the world because you need developer rights.
If you accidently modify and existing entity, get the error and can't load the main or your mod after that: Just remove the Main folder in the mods folder.
Summary:
- You can't edit official main mod entities
- You can add your own entities to the official main mod
- If you can't load the main mod anymore: try to remove the Main mod folder in your mods folder.
Again, thanks to BlazeDragoon for telling me this!

edit:
I already tried to add a similar guide to the wiki but I got instantly banned for: "Large spam page" o.O