gpart create -s mbr ad0
Install boot code to the mbr - basically setting the boot flag for the disk
Note: that the /boot/mbr directory must exist on your existing OS (I'm using FreeBSD 6 diskless)
gpart bootcode -b /boot/mbr ad0
MBR/Root Partition - slice
gpart add -s 800M -t FreeBSD ad0
Set it to active so FreeBSD will know to boot off of this specific partition
gpart set -a active -I 1 ad0
BSDLabel Partitioning - sub-mbr partitions - / - root
gpart create -s bsd ad0s1Add boot code to the sub partitions as it needs to have a boot flag as well to know it can boot off this drive
gpart bootcode -b /boot/boot ad0s1
Add partitions to the MBR/root
gpart add -t FreeBSD-ufs ad0s1
Create the FS structure on root with the UFS label of ROOT
newfs -L ROOT -U /dev/ad0s1a
Mount the root partition
mkdir -p /mnt/root
mount /dev/ad0s1a /mnt/root
Now you can un-tar a filesystem to /mnt/root!
cd /mnt/root fetch ftp://path/to/file.tar tar -xf file.tar rm file.tar
BSDLabel, Slices and MBR (Dangerously Dedicated)The only reason that I have seen to create slices is if you wish to maintain compatibility with MBR based OSes (such as windows). FreeBSD will commonly encapsulate bsdlabel inside an mbr slice.
If you're running on FreeBSD only machine you do not even have to use MBR slices and can use bsdlabel directly which would result in partition names such as ad0a, ad0b, da0a instead of ad0s1a, ad0s1b, da0s1a.
The mode in which you do not maintain compatibility with MBR is called "dangerously dedicated" mode but it is perfectly acceptable as long as you do not need to maintain MBR compatibility for anything.
I've also recently heard that in FreeBSD8 and 9 they have removed the ability to configure devices in "dangerously dedicated" mode.