This article provides a step-by-step guide on how to set up jails on FreeBSD 12 without any helpers. All jails are placed under /var/jail and given their own directories reflecting their short hostname. You can either create one big /etc/jail.conf or one /etc/jail.HOSTNAME.conf per jail. To have the jails started and stopped during system boot and shutdown, add the jail_enable and jail_list to /etc/rc.conf. In addition, to give your jails Internet access, NAT is required on the host, and a local unbound as DNS resolver can be set up. Finally, the article provides instructions on how to install the ports collection, and how to enable sending emails via the host system.
https://ogris.de/howtos/freebsd-jails.html