you use the installer, install the base system
log on
go to shell (8 from console)
use fdisk to partition the other disks (google fdisk)
create the filesystem (google how to do this, i forgot the EXACT command)
you're probably better off just putting the entire "var" directory under a single disk…it's going to be easier in the long run
to do this just make a new directory
mount your new disk (again, google mount if you need to but it's basicly mount DEVICENAME DIRECTORY NAME so if your device is /dev/ad9s1 and your new directory is /mnt/newvar the command would be mount /dev/ad9s1 /mnt/newvar
copy everything from var to the new drive using the cp -r command
use the mv command to mv the old var to /var-old (you do this incase something goes wrong and you can easily restore it)
use the cp command to cp /etc/fstab to /etc/fstab-backup
create a NEW /var directory with mkdir
edit fstab to mount your new /var disk
that's the method on a system ALREADY installed...i might have left some stuff out.....it's been awhile since i've done this in free bsd
on a new install you can probably use the installer to partition and mount disks...