We’re planning to tile the tub surround in the basement bathroom. The first step was to install Hardibacker board to the studs (with the correct special screws). I’ve read a variety of methods for configuring the backer board and the flange of the tub, and ultimately I just screwed the board to the studs and left a small gap above the flange. I believe the idea is that by leaving a gap, there’s enough thinset to hold together and not crumble. I put fiberglass mesh tape on all the board seams, including across the gap above the flange, figuring it would also help hold it together. Then I thinset over the seams, the screws, and the flange completely, to make a nice even substrate for the tile.
I didn’t put any vapor barrier behind the board. Instead, after the thinset cured, I applied two coats of RedGard over the board to create a waterproof surround. In theory, the water should never get into the wall system this way, but if it does it should still be able to dry. I haven’t drywalled the inside of the mechanical room yet. My intention is to thoroughly test the shower for leaks before we insulate and drywall the other side of that wall. I highly recommend the plastic sheeting over the tub; it caught more than a couple drops. The next step for the tub is tiling, but before that I need to finish mudding the drywall.