Which games support it
Automated Steam Workshop installation, through the Mods tab, is available for: ARK: Survival Evolved, Arma 3, Conan Exiles, DayZ, Don't Starve Together, Squad, and Unturned.
Not every Steam game with Workshop support in general has it wired up here. ARK: Survival Ascended, CS2, TF2, Left 4 Dead 2, Arma Reforger, and several other titles do not have an automated mod browser — install mods manually via the Files tab instead. Garry's Mod uses a different flow entirely (see its own Getting Started article).
Browsing and installing
Open the Mods tab and use the Browse Catalog view to search Workshop items for your game. Selecting Install queues the download — switch to the Overview tab and watch the server console for live download progress.
Managing installed mods
The Installed Mods view reads directly from the server's Workshop content folder, so what's shown there always matches what's actually on disk. Removing a mod from this list deletes it from the server.
Games that require stopping the server first
Arma 3 and DayZ require the server to be fully Stopped before you can install or remove Workshop mods. Mod "keys" are handled automatically for these two games when you install or uninstall.