Requirements
You can install mods only on Forge servers and modpacks.
Spigot, Bukkit, Standard Minecraft don't support mods.
Mod Files
To install mods you will need to get .jar files.
Search and download Minecraft Server mods here:
https://mods.curse.com/mc-mods/minecraftAs an example, we will describe the JourneyMap mod installation process:
https://www.curseforge.com/minecraft/mc-mods/journeymap.
Files Upload
In order to upload your mods go to the
Your Gameserver → File manager page, open folder "mods" and click Upload.
Reboot the server to start using the new mods.