How do I register a currently unregistered 2002 caravan in France?

We have a Hobby 540 UK caravan that we bought new in Holland in 2002 and drove it around for years with English plates on the car. It is now in France where we now live and we have a French buyer for our caravan, so we want to explain to him what he needs to do to register it. Has anyone had experience of doing this on a caravan of a similar age?