My first: Add-on

For creating and publishing an add-on in The Divinity Engine 2, please read Working with projects.
Below, we detail any extra requirements to get an adventure working in-game and how to check if it works as intended.

Everything needs a good story

Enabling add-ons will trigger the engine to build a story based on all elements from the campaign and all enabled add-ons.
To ensure your add-on will work with the game, build the story in the toolset before going into game.
Don't worry, you don't need to invent some story scripts if your add-on doesn't do any story things. Every project has a story base that's automatically included on creation. It even gets built automatically on publishing.
Only when testing locally, before publishing, you'll have to trigger a build yourself.

More information on the story editor can be found here: Story editor

My add-on in-game

External mods (f.e. from Steam Workshop) are always placed under "Documents/Larian Studios/Divinity Original Sin 2/Mods".
But your own mods are magically loaded as well! So close the Divinity Engine 2, launch the game and check it out!

Story:

Game Master Mode:

That's it! The description shows you which modes were set as target in the editor. These can be changed in the Project settings window.
Now the add-on will be automatically enabled for every game in those target modes!

ATTENTION: The order in which add-ons are enabled determines which change gets priority if multiple add-ons change the same thing!

For more information on the modding system and mod priority, check this page: Modding internals