Peel off previous carpet, cladding or non-stick paint. Rub the varnished surface with sandpaper, to increase its grip. If the old layer of ceramic tiles sticks well to the substrate, you can place new tiles on it. If you prefer to remove the old glaze, use a chisel and a wooden hammer or a small engraving hammer. If necessary, you can also align the exposed surface, by applying gypsum or plaster on walls and ceilings, be – on the floors – chipboard or plywood.
It is very difficult to lay the tiles evenly, if the wall or floor is not perfectly even. You can examine the ground, by applying a long one in different places and in different directions, level board and checking, whether there are wall or floor unevenness.