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.
Tiling walls that are not plumb.
Slide a 4 foot straightedge or level over the wall in all directions to check for plumb.
Lippage can be eliminated on most walls by hanging new drywall or backerboard before tiling.
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.
Check the wall first to see if it is straight.
Subway tiles are notorious for not being well made so i always tile all 3 walls at the same time so the grout joints match up on adjoining walls.
If you re tiling directly onto the wall however leveling clips and wedges will force the edges of the.
Its logical to do one wall at a time but it doesn t work out that way.