Well it is not that difficult. You must cut a piece of zipper tape, it must be a bit longer than the opening of the bag, close it at the end with a sort of leather "tail", then cut out four leather swipes of leather (1 or 2 cm wide) . The zipper tape must be sewn between the swipes each side. The zipper must be sandwiched between the facing and the lining. It's the falling lining technique. Probably it would be a bit easier to understand if you could see the procedure. I don't know if I was clear enough.... Be patient!