Many tile installers put the backerboard on before installing the mud base, which is acceptable if the board is kept off the surface of the pan—where it could wick up moisture and cause discoloration of the tiles. But this method requires the installer to step on the membrane, potentially exposing it to damage. Instead, wait until after the mud base is installed to install the backerboard, so that you never have to step on the membrane. With this method, the board can sit directly on top of the mud layer. Attach the board with 1 1/2 -inch galvanized roofing nails, keeping all the nails at least 2 inches above the top of the threshold.
If there are areas where the tile will overlap onto a finished wall surface—like the sides of the shower entry on this project—brush a liquid waterproofing membrane, such as Hydro Ban, on the overlapping area.