Use tile spacers if necessary to keep the tile lines straight in the layout.
How to tile a wall that is not plumb.
You can remedy this by making sure that the tiles are positioned so that no slivers are used.
Answer answer what should have been done while installing the hardibacker board on the walls is the framing studs should have been shimmed and adjusted to make sure the hardibacker board was plumb and square.
The back of the tile should be flush with the wall.
Spread the mortar with the flat edge of the trowel then groove.
If the walls were not too far out of wack you can use the thin set mortar or there are special patching mortars for going over the hardibacker board as long as it isn t applied too thick.
Use no tiles less than half wide along the outside of your tile field any tile slivers tiles that are less than half the width of the tile itself will markedly show the out of plumb wall.
Start applying the adhesive in a bottom corner about 1 tile length away from the bottom and side of the wall leaving space for the edge tiles.
Scoop out a golf ball sized amount of adhesive onto a notched trowel and spread enough adhesive in a thin layer over the wall to hang 2 3 tiles at a time.