Pretty simple, the best thing to do is to proceed like in the FAQ above (How can I do automatic backups?) and just insert a command for changing the tape in the shell script. So before calling KBACKUP, you would just let the auto-loader change the tape and then start the backup. Unfortunately, then you can only do backups that fit on a single tape.
If you want to write longer (multi-volume) backups, you need to know MULTIBUF how to change tapes. In the Configuration-Menues you find an entry for the commands MULTIBUF will execute at the end of a tape. Here you need to enter a command for changing the tape, and everything should work fine.