First you need to choose the right server type. Follow the guide here
1. Log in to the Game panel
2. Choose the server you want to install mods on
3. Find the mod you want to install, you can do this on Modrinth or similar
4. Make sure the mod you download matches the server version and server type
5. Download the mod
6. Click Files in the Game panel
7. Open the mods folder
8. Upload the mod to the mods folder
7. Start the server
The mod should now be installed.
It is your job to make sure the mod matches the server type and that it fits the version you have chosen
