the only way to reliably tile or existing tiling is to provide a mechanical key for the tile adhesive. Glazed / fired tiles will not provide such a key so you have 2 options – prtially grind the surface as described above, or apply a coat of Ammonium biflouride which will etch into the glaze and make it suitable for tiling.  Be careful of this chemical though, it is quite toxic.  The tile adhesive to use should be a polymer modified one [either acrylic or SBR]  PVA is not reliable.

I have 2 bathrooms to do and I will use the chemical technique. Quick, easy, no mess [but dangerous – you have to be caareful]