The points made about the characteristics of the floor foundation surface can also be made about the roughness of the plaster (if you don’t want to have recourse to special additives in the adhesive) in order to have an excellent fixture of the base of the tile.
A simple trick allows tiling to be accelerated and guarantees that the tiles are perfectly horizontal; just place a piece of string of the desired thickness between one tile and the other. Verticalness however is guaranteed by using a plumb line in alignment with the modular grill of the floor. The tiler’s experience is however at the base of the result of the tiling.