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Measure and mark for height of chair rail so it falls between the niches. Transfer that measurement to the other walls.
Place a few tiles against the wall and check for level. If the floor is not level, you can shim the tiles and build up the grout line.
For a standard brick pattern, use a level to make two vertical reference lines. Set spacers on the floor for a grout line.
Back butter the tile with thinset using a 1/4" x 1/4" square-notched trowel (Image 1) and set tiles working your way up to the chair rail (Image 2).
Back butter the chair rail corner pieces and set in place (Image 1). Find center point between the two pieces, then center the first chair rail piece on that mark (Image 2). Set the chair rail working out toward the corners (Image 3). Any pieces that need to be cut will then be the same size on both sides.