The secret is getting a good bond to the old tiles, I would acid wash them, hen a few more washes and then get a product which Bricoman sell that is a polymer, you mix it with a bit of sand and cement and either roll or brush it on. It will bond well and also form a waterproof layer. What ever you do, do not use a PVA because, if for some reason it gets some water through the grout, it will reconstitute and the tiles will become drummy. PVA is not a waterproofing agent.
If the old tiles are solidly fixed then all you have to do if buy some special undercoat. I can't for the moment remember what it's called but it's easy to find in any good dyi place like Casto or Leroy Merlin where in any case you'll find someone who'll advise you. It''s a sort of glue I reckon -- paint it on, let it dry and then tile on top. I've done this in two bathrooms, walls and floors, and after 16 years there's no sign anything is about to fall off!