Secure the Backer Board to the Wall
Because the plywood has ridges and seams, a backer board had to be put up first before applying the thinset, and with plywood being on the wall, they only needed 1/4" backer board. Backer board can be cut with a utility knife but needs to be scored first using a straight edge as a guide. After you score the board, you just snap it like drywall.
Note: Backer board is made from cement and won't rot or swell when exposed to moisture. You can make fine cuts in the backer board with a jigsaw.
Before you put the backboard on the wall, apply a bead of construction adhesive. This helps create a positive bond so there is no void that would cause a flex. Push the backer board gently and easily into the adhesive.
Screw the board to the wall using special screws that have ridges under the head. These cause the screws to automatically countersink into the backer board.
Be aware of pipes in the wall. Find their location before you start.