If you search these forums you will find the answers, (although not a real HowTo)
and to help a little more here is one I wrote earlier, If you are using SME5.6 please read the later messages in the thread as it needs a slight change.
http://forums.contribs.org/index.php?topic=5421.msg19122#msg19122I would also recommend that you check
/etc/e-smith/web/functions/backup
to see what the Server Manager - Backup to Desktop does, and to confirm that the cron script matches this.
I have now tested all SME 5.x versions and it works for me - but I advise you test this to ensure that it matches your needs.
Ian