Find the hosting.cfg file within the FTP client and open it with a text editor. We strongly recommend Notepad++ , which can be found here: https://notepad-plus-plus.org/
Within the hosting.cfg file, add steam_ugc= to a new line
Head to the Steam workshop and find the mod you wish to install. You will need the mod's ID number, which can be found at the end of the URL.
Paste the MOD ID number after the = with each mod being separated by a comma.
Save the edits and re-upload via your FTP program. You will be prompted to overwrite any changes, which you need to choose "yes" or "confirm" to.
Start the server back up via the control panel.
If you have any issues with this method, please let us know.