To start, boot From install CD, Choose live CD option when prompted. You will be presented with a login prompt, use root as the username, you will not be prompted for a password as there isn't one set on the live CD.
The next step, is already your point of no return, this one will erase everything on the hard drive and create knew empty partitions. As FreeBSD is used by many for servers, and while working on this my goal was to create a more easily upgradeable server install to test upgrades and have an easy way out if the upgrade broke something, the partitions are going to be created using 2 hard drives and a mirror. This adds some hardware redundancy to the setup.
On my test system I created the documentation on the two hard disk were SCSI drives, found by the system and created as devices da0 and da1, replace those with the appropriate disk devices for your system. In my case I know the drives are blank as they are freshly created virtual disks, however if they are not the best place to start is to first destroy any gpt partitions that might exist on the drives.
gpart destroy -F da0 gpart destroy -F da1
Note: if you receive "gpart: arg0 'da0': invalid argument" as an error, it means that there was no defined gpart partion table on the device, you can safely ignore this error and continue, if you are sure you have the correct device.