Setup: Editor Steam

1. Enable DLC

The editor needs some extra data. For this you need to enable a DLC for DOS2 on steam called 'Divinity Engine 2 Data'.

2. Install toolset

Open your library and go to the tools section. There you will find 'The Divinity Engine 2'.

3. Initial setup

On starting up the divinity engine 2, it will ask for a game data path. Set this to your Divinity: Original Sin 2 'Data' folder.
It looks approximately like this for steam: 'C:\Program Files (x86)\Steam\steamapps\common\Divinity Original Sin 2\Data'.