Setup: Editor Steam
1. Enable DLC
The editor needs some extra data. For this you need to enable a free DLC for DOS2 on steam called 'Divinity Engine 2 Data'.
Go to your steam library, right click Divinity: Original Sin 2 and select properties.
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'.