Just after the install is done, but before you exit the installer, run these steps:

1. Switch to the Rescue TTY via Alt+F4.
2. Run csh to get a shell with command history.
3. Run sysctl kern.geom.debugflags=16.
4. Run: gmirror label -v -b round-robin gm0 /dev/ad2.
5. Add geom_mirror_load="YES" to /boot/loader.conf.
6. Open /etc/fstab in vi and run :%s#ad2#mirror/gm0#
7. Switch back to the installer with Alt+F1.
8. Exit the installer and reboot, removing the CD you installed off of.
9. Login as root and sync the disks with: gmirror insert gm0 /dev/ad3
10. Watch the build with gmirror status, it will look something like this:

          Name    Status  Components
    mirror/gm0  DEGRADED  ad2
                          ad3 (4%)