The bsdinstall(8) tool can be used to fetch and install the binaries needed for a jail. This will walk through the picking of a mirror, which distributions will be installed into the destination directory, and some basic configuration of the jail:

# bsdinstall jail /here/is/the/jail

Once the command is complete, the next step is configuring the host to run the jail.

To install the userland from installation media, first create the root directory for the jail. This can be done by setting the DESTDIR variable to the proper location.

Start a shell and define DESTDIR:

# sh
# export DESTDIR=/here/is/the/jail

Mount the install media as covered in mdconfig(8) when using the install ISO:

# mount -t cd9660 /dev/`mdconfig -f cdimage.iso` /mnt
# cd /mnt/usr/freebsd-dist/

Extract the binaries from the tarballs on the install media into the declared destination. Minimally, only the base set needs to be extracted, but a complete install can be performed when preferred.

To install just the base system:

# tar -xf base.txz -C $DESTDIR

To install everything except the kernel:

# for set in base ports; do tar -xf $set.txz -C $DESTDIR ; done

The jail(8) manual page explains the procedure for building a jail:

# setenv D /here/is/the/jail
# mkdir -p $D      
# cd /usr/src
# make buildworld  
# make installworld DESTDIR=$D  
# make distribution DESTDIR=$D  
# mount -t devfs devfs $D/dev    
Selecting a location for a jail is the best starting
	  point.  This is where the jail will physically reside within
	  the file system of the jail's host.  A good choice can be
	  /usr/jail/jailname,
	  where jailname is the hostname
	  identifying the jail.  Usually, /usr/
	  has enough space for the jail file system, which for
	  “complete” jails is, essentially, a replication
	  of every file present in a default installation of the FreeBSD
	  base system.
 
If you have already rebuilt your userland using
	  make world or
	  make buildworld, you can skip this step
	  and install your existing userland into the new jail.
 
This command will populate the directory subtree chosen
	  as jail's physical location on the file system with the
	  necessary binaries, libraries, manual pages and so
	  on.
 
The distribution target for
	  make installs every needed
	  configuration file.  In simple words, it installs every
	  installable file of
	  /usr/src/etc/ to the
	  /etc directory of the
	  jail environment:
	  $D/etc/.
 
Mounting the devfs(8) file system inside a jail is
	  not required.  On the other hand, any, or almost any
	  application requires access to at least one device,
	  depending on the purpose of the given application.  It is
	  very important to control access to devices from inside a
	  jail, as improper settings could permit an attacker to do
	  nasty things in the jail.  Control over devfs(8) is
	  managed through rulesets which are described in the
	  devfs(8) and devfs.conf(5) manual pages.