Subscribe
Navigate to the Steam Workshop and subscribe to Black Mesa: Damocles. Click the button below to visit the workshop in your browser or this Link to open the item in Steam.
Link with your Game
To play the mod, you need to make it accessible for the Black Mesa main game. After steam has successfully downloaded the workshop item, you will find the files in the directory steamapps/workshop/content/362890. For Windows users there is a damo_gamelink.bat and for Linux users a damo_gamelink.sh script, which create a link in your Black Mesa game directory. Alternatively you can copy the whole damocles mod directory into the Black Mesa directory.
Adjust Launch Options
The final thing to do is to start the game with the option -game damocles. Right click on Black Mesa in your library and choose Properties and click SET LAUNCH OPTIONS.