Create user using sudo adduser <user>. If you are using a custom location, use sudo adduser --home </<path>/<user>/ <user>
Enable linger on the account with sudo loginctl enable-linger <user>
Enter the account using sudo machinectl shell --uid=<user>
Create folder $HOME/.config/containers/systemd/ if needed
Create container file <application>.container in $HOME/.config/containers/systemd/
Create network file <application>.network in $HOME/.config/containers/systemd/
(optional) Create volume file <application>.volume in $HOME/.config/containers/systemd/
Run systemctl --user daemon-reload to create the systemd services
Check that the services have been created with systemctl --user status <application>. The network service is <application>-network.service and the volume <application>-volume.service