This topic has been raised within the pages of this Forum and a search would allow you to see the results. However, you need firstly a polarity checker ( available from most caravan accesory shops). You also need a short cable with a French Plug at one end ( again available in caravan shops, or a hypermarket in France). The other end should have a standard blue socket to attach to your standard caravan cable. The short extension cable should be wired in reverse order, that is the blue to live and the brown to return. Then if your polarity tester shows a reverse situation, you then insert the reversed cable and all should be well. If your situation is normal as per the polarity tester, then reverse the wires in the French plug to the correct (brown to live, blue to return) and again all will be well. Some people try to change the blue connecters on their main 25 meter cables, not a good idea as you will probably forget where you are and arrive back in the UK 'wrongly' connected.
It is more complicated to describe than it actually is.