Fabric Server From A Fabric Pack

Step 1: Creating the server

Create a folder somewhere that will be the server folder.
Open the pack folder.
Select everything in pack folder.
Now copy that selection. For example right click the selection and choose copy.
Open the server folder you made in step #1.
Paste the previously copied selection. For example right click and choose paste.
Download the installer for Fabric. This one installer covers all versions from 1.14.4 to now. The latest installer can always be downloaded from fabricmc.net/use/.
Move this installer jar to the server folder
Open your shell/terminal/cmd/powershell
Enter the server folder you created, you might have to navigate to it in your file manager.
In the following you can replace 1.16.5 with whatever version of Minecraft your Fabric is designed to use. Enter the command java -jar name_of_installer.jar server -mcversion 1.16.5 -downloadMinecraft
A bunch of stuff will happen
At this point the server is created but not debugged
I recommend deleting installer jar as you don’t need it anymore.

In your shell get a directory listing of the jars (Linux: ls *.jar Windows: dir *.jar)
If you haven’t done so already, delete the installer jar.
There will be a new jar file that doesn’t say installer and starts with “fabric”, this is the file you will be executing, note it down.
From your shell to start the server do java -jar fabric-server-launch.jar
You can add nogui to the end to stop the gui from popping up.
The server will start then stop, telling you edit the eula.txt. so that and set the one value there to true.
Start the server again.
The server has started.

The server you created will likely crash because quite a few mods are not server friendly or coded in such a way that they will try to load client side classes on a server. When it happens there will be a crash report. Open the file and look at the top trace block to see what crashed.
Check to see if there is an update for the mod, if yes, update it on both the server and the client, rerun the server
If the server crashes again on the same mod, try a lower version, make sure the client matches
If it crashes again, remove the mod from both the server and the client
Eventually you’ll have a running server and client that should connect to it

Great job on creating a functioning server
now run the client that was adjusted alongside the server
if it crashes look at the crash report and either update, rollback or remove the mod from the client and on the server if it exists
eventually you’ll have a running client