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.
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!