Making a backup with tar is done by typing something like that on bash:

> tar -c - dir1 dir2 dir3 > /dev/tape

That will (hopefully, I doubt I got the tar switches right) backup those dirs into the tape (that will actually have a weird name, not '/dev/tape').

Now, in practice Linux doesn't always know the size of a tape you inserted. But this is not the issue, if you accept the seeks needed for that, you'd better write at the beginning anyway.