You really can't. The bit your bum sits on - pull it up suddenly and hard, then pull up your read headrests all the way out. Then tilt your rear backrests out to reveal... a metal bulkhead! Such a shame as I'd love to. On mine (is it the same on yours?) a 3-point rear centre seat belt was only available with folding rear seats.
If you manage to do it, or want to have a go, let me know and I'll come and give you a hand... then you can help me do it on mine!
(What kind of manufacturer doesn't have folding rear seats as standard!)