Given that this isn't something I do regularly, it's handy to have the commands to hand so I don't have to scroll through man pages trying to remember how to do it.

Clear the disk

# dd if=/dev/zero of=/dev/ad1 bs=1k count=1

Initialise it

# fdisk -BI ad1

Label it, then edit the labels created

# bsdlabel -B -w ad1s1 auto
# bsdlabel -e ad1s1

Format it with a filesystem

# newfs /dev/ad1s1e

Make directories and mount as appropriate, and you're done!