If its like the ones on my old 306's:

1. Remove the plastic cap to reveal the socket on the towbar into which the
tow ball assembly fits

2. There is an internal spring loaded bolt which secures the tow ball
assembly in place while in use. You will need to unscrew it using the little
lever to one side of the tow bar socket - rotate this lever anticlockwise
until you can pull this bolt back

3. Offer the towball up to the towbar socket upside down, it should then
slide in. Rotate it right side up, you may feel it click into position as
the tip of the retractable bolt lines up with the hole in the side of the
towball shaft. Release the bolt lever and retighten to secure the tow ball
in position.

4. Removal is reverse of the above. Strictly speaking, it is a legal
requirement to do so whenever driving solo, as the tow ball obcures the rear
number plate on the models where this type of towbar is normally fitted.