First go onto the Steam Workshop and search for mods you want to install.
Then you have to get the ID for that mod, which is the numbers at the end of the url of the mod.
E.g. for Valhalla which has the mod id 504122600 you would find it here in the url: https://steamcommunity.com/sharedfiles/filedetails/?id=504122600
Next, go onto the control panel and click on install mods, paste in the id's of the mods you want to install separated by commas - with no spaces.
Click execute, and the script will install the mods one by one and add them to the ActiveMods list.
Once the mods are installed by the script, click restart on your server and they should be installed!
You can also install mods by adding them into the ActiveMods list in the UserGameSettings.ini and then clicking on Update Mods to download and install.
To update mods, simply go onto the control panel and click on the Update Mods button. You can also check for mod updates by clicking on the Check for mod updates button.
To remove a mod from your server, you can use the Uninstall Mod button and enter the mod(s) IDs that you want to remove, then click Execute. You can also edit the ActiveMods list in the UserGameSettings.ini and just delete the mod id from there to disable it from the server.