There are a number of things that have to be done to prepare the floor. A cross section of the floor you need would have a thin layer of cement over the main plywood floor. It is very important that the cement tapers down from where the floor and wall meet to the drain. Over the cement you have to install a waterproof plastic membrane. It should extend from the drain, across the floor, and up the wall 10 cm (4 in.) or so. If a leak later develops on the bottom of the wall, it cannot penetrate beyond the tiles. Above the plastic membrane there should be another thin layer of cement over which the tiles can be set. If you do not want to dig up the floor, you can build the same layers of materials on top of the floor.