tub should have came with a cut out template which also shows the drain location. frame first knowing your height, add an inch for mud, and width for shelves/sides. use a laser to frame level. sheath with 3/4 ply and 1/2 cbu. mark and cut template opening. have shims ready for thickness of tile and mud. whatever you use for mud base leave it thick with hardly any slump and put as needed. set tub down to shims. connect the drain and put some water in. shake tub down to shims. do your supplies and sheath, rock, cbu the sides.