Starting on FTBs website, we can find the server files for Skyfactory 3 by searching skyfactory 3.
After selecting Skyfactory 3, there will be a download option and an install script option.
I used the install script. The command to get the script looks like curl -JLO "https://api.modpacks.ch/public/modpack/25/123/server/linux" && chmod +x serverinstall_25_123.
Before running the script, make a directory for the server. Mine is at ~/MinecraftServers/Skyfactory3. mkdir ~/MinecraftServers && mkdir ~/MinecraftServers/Skyfactory3 && cd ~/MinecraftServers/Skyfactory3, then run the command to get the install script.
Next run the install script, ./serverinstall_25_123. Keep entering yes to the questions asked, then within a few seconds, your server will be installed.
There will be quite a few files and folders created. The most important file is the start.sh script. Run that to start the server, ./start.sh. The first time it runs, you will be asked if you agree to Minecrafts EULA. Just enter y for yes.
It might take a few minutes for the server to initialize. Now go ahead and pop open your skyfactory client and have fun!
Screen is a utility that can be used to run processes in the background so you can disconnect from the computer and the server will still be running.
Use sudo apt install screen to install.
Then just use screen to open an instance.
After starting the server use ctrl+a, d to detach.
To resume the session, use screen -r. This won't work if there are multiple sessions so use screen -ls to find the session id. Then screen -r id will get you into the session.