DIRECT ANSWER
Open the Steam Workshop, subscribe to the item, let Steam finish downloading, then verify that the current build supports that mod type before loading a save.
Install checklist
Workshop presence does not by itself guarantee current in-game support.
- Read the item description and required version
- Subscribe in Steam Workshop
- Wait for the Steam download to finish
- Launch the game and check the relevant mod menu
- Test in a disposable scenario or save
- Unsubscribe and verify files if the build becomes unstable
Early Access launch warning
The official Steam listing says modding will not be available at the Early Access launch, though integration is planned soon. Recheck the update tracker before expecting subscribed items to load.