Determine the size, shape, height and location for the bench, once the basic shower enclosure has been designed and built. At the same time, determine what the height of the mortar bed will be at the location of the bench. Add the height of the mortar bed to the desired finished height of the bench (for instance, if the mortar bed is going to be 2 1/2 inches at the outer perimeter of the shower where the bench is located and a 20-inch-tall bench is desired, the bench should be roughed out at 22 1/2 inches).
Frame out the bench as desired. Build a slight slope into the bench (approximately 1/2 inch per 12 inches) to allow for water run off. Remember to deduct 3/4 inch from the framed height to allow for the plywood top. Deduct an additional 1/2 inch from the framed height to allow for the tile backer board as well.
Once the bench is framed, install the plywood rough top. After the rough top is installed, install the pan liner or moisture proofing along the bottom of the shower pan as you would have for a normal shower pan. Also install the pan liner or moisture proofing along the top of the bench (including at least 6 inches on the walls above the bench) overlapping the shower pan liner. In essence, the bench has just become an elevated portion of the shower pan.
Install the backer board throughout the shower and prep for tile installation, remembering to install backer board from the bottom up so that the upper pieces overlap the lower pieces on the bench.