These are more complicated to fit than a normal door lock. Normaly the locks do come with instructions. I have sat here for a while trying to explain in text how its done but im finding it hard. Can you look at someone elses french doors to see how it looks when it's fitted so you have an image in your head how it should look when finished? Main point is that on a normal lock the lock plate should be flush with the door edge. On french door you will have a L shaped lock plate, you need to ensure it will finish so both sides of the face plate are flush. If one side isn't and sticks out from the door the doors won't shut.
May be worth getting a joiner to fit for you, you will probably be only looking at about £15.