I get a LOT of questions about how to build and waterproof a bench in your shower. I’ll touch on the easiest method here, but there are a couple of different methods you can use.
I will describe simple framing of a bench with your substrate over it. You can also use after-market, pre-fabricated benches. Better Benches (google it) attach directly to your wall substrate, the top gets filled with deck mud and it gets tile. There are also several different Styrofoam products available from companies like Schluter and Laticrete. They are made from the same type of foam used for their shower bases. Although they are ‘foam’, once tiled they are more than sturdy enough to support your tile.
While you ‘can’ build a bench in your shower after you form the shower floor with deck mud, it’s always easier to make your bench first. Your floor substrate is flat, your shower floor (should be) sloped. It’s difficult to build a level bench on a sloped floor.
But you can do it if you wanna.