Mark the First Diagonal Tile
On a subfloor of cement backer board screwed in place, dry-fit the first tiles on the floor before getting started. Position the first tile in a corner of the room where the walls make a 90-degree angle. Lay the first tile so that one of its flat sides is toward the corner and the tile's corners are almost touching the walls. Use a speed square and measuring tape to make sure the tile is centered and sitting at a 45-degree angle to the walls. Mark the floor along the top and bottom edges of the tile for later reference. From that first tile, lay out the design across the rest of the floor and mark the floor with parallel lines so that tiles can be accurately placed.