Hey I wanted to install Fabric on on of my servers. The one article for help in here did help me in understanding how things work but ultimately didn't work for me. But in the end i did figure it out and thought of sharing it.
1. Download the Fabric installer from their website
2. Open the .jar file and switch in the top section to server, there select the minecraft version that you want to play on
3. Click on Install. That will create a fabric-server-launcher.jar file and a folder called .fabric-installer
4. The folder you can just delete. Don't need it anymore
The following expects you to have Minecraft (Vanilla) on the server installed for the same version as you selected in step 2.
5. Go to the file browser section in the Nitrado interface
6. Open the minecraft folder, and ename the minecraft_server.jar file to server.jar
7. Upload the fabric-server-launcher.jar to the same folder where the server.jar is located
8. Rename the fabric-server-launcher.jar file to minecraft_server.jar
9. Restart the server. (It might take a minute or two for minecraft Fabric to really start, so be a little patient)
Fabric is now installed. Congratulations.
To install mods:
1. Go back to the file browser
2. Open the minecraft folder and then the mods folder (if it doesn't exist create one or start the server and come back in a minute when the server is running fabric creates the folder for you)
3. Upload whatever fabric mod you can imagine