I would like to tile (with normal porcelain tiles) our hallway which currently has quarry tiles. The floor sinks slightly in the middle so we decided to investigate and lifted a few quarry tiles out. The subbase appears to be sand which is what I expected after reading a few websites on the internet. The area is about 1sqm so quite small and manageable for any work. What do I need to do to prepare the surface for tiling? The rest of the house is floorboard. Many thanks.