Would the wall and the tile both need to be fully buttered with mortar to ensure adhesion?
Buttering could be necessary if the surface is uneven. Or in this case giving the size of tile.
I would take Nicholas from Classic Homes Suggestion and use a leveling spacer system.
Back buttering any size tile is a good idea, especially large format tiles like yours. It just creates a better bond to the tile.
With your 12x24 tiles you should use a 1/2" notched trowel on the wall and back butter each tile, but just with a skim coat. If your walls are flat enough, this should give you the wiggle room you need for a flat surface. If don't have a lot of experiance tiling, then you should use a tile leveling system. There cheap and for a novice useing large tiles, an absolute must.
Also if you are using tile spacers that a little flex to them, then only tile half way up the wall per day. The weight of the tiles will crush the spacers and your grout lines will be uneven.
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