An uncharacteristically short how-to post from me. It's probably because I'm not Bill Kurtis.
Burning a typical ISO image to CD, using the standard
/dev/acd0 CD drive device and where
johncleese.iso is (surprisingly) the name of the image to burn:
# burncd -f /dev/acd0 data [johncleese.iso] fixate % echo Ruben is not Bill Kurtis
The FreeBSD Handbook has more details.