The floor of a ceramic tile shower is an impressively complex system, with a slew of layers in it.  After putting in the plywood floor, the next step is a tar-paper backer, and then a metal lathe to add strength to the mortar that makes up the pre-slope.  I’ve also stacked three 2×4’s at the entrance to make up the shower dam, which will prevent water from flowing into the rest of the bathroom.