Laying tile at a 45 degree angle is sometimes called a diagonal layout. Instead of the joints and grout lines running perpendicular to the walls, they run on a 45 degree angle. The look adds visual interest to the room and, once the floor is properly laid out, laying down whole tiles moves along quickly.
Clean the subfloor -- concrete, plywood or existing floor -- to remove old glues, grease, paint, any substance that might inhibit how well adhesive bonds tiles to the subfloor. Check the subfloor and make sure it's flat with no nail heads, screws or other material sticking up.
Measure the dimensions of the room to determine the width and length. Make a pencil line to mark the centre of the room lengthwise and another to mark the centerline widthways. The intersection of these two pencil lines should break the room into four quadrants, each meeting at the exact centre point of the room at a 90 degree angle.
Lay a framing square so that one leg crosses a pencil line at the 12 inch mark of the square and the other leg crosses at the 12 inch mark on an adjacent pencil line. The point of the square should be aimed away from the room's centre point, so that the two pencil marks and the two framing square legs create a box. Mark the inside of the box, then use the square to connect the opposite corners of the box. This line is a 45 degree angle inside one of the four original quadrants. Make the same pencilled layout lines for the other three quadrants.
Snap chalk lines along the pencilled 45 degree layout lines. The floor of the room should end up with a large "X" made up of two chalk lines that intersect at the room's centre point.
Dry-fit several tiles on the floor to get a sense of how the tiles will look in the room. Dry-fitting is setting down tiles without adhesive. Use the chalk lines to represent grout lines as you set the tiles in place. Use plastic spacers between tiles to represent grout lines.
Lift those dry-fitted tiles and spread tile adhesive on the floor with a notched trowel. The adhesive dries quickly, so put down only the amount that you can comfortably work with in about 30 minutes. Work one section at a time, between the chalk lines, but be careful not to cover up the lines with adhesive. Rake the trowel across the wet adhesive to create ridges that help tiles set and bond correctly.
Lay tiles onto the adhesive beginning with the centre of the room. If the tiles are over 1/8 inch thick and/or larger than 12 inches by 12 inches, butter the backs with tile adhesive before setting them into place. The corner of the first tile should point to the centre of the room and two adjacent edges should line up with two adjacent chalk lines. Wiggle the tile about 1/4 inch or so to ensure good adhesion and make sure the tile lines up correctly between the chalk lines. Tap tiles gently with a rubber mallet to seat them.
Place each tile into the adhesive, one at a time, and put plastic spacers between them to ensure the proper spacing. These spacers will help maintain straight grout lines and keep the diagonal layout lined up correctly as you work the room.
Check tile with a level periodically to make sure none are sticking up too high or resting too low. High tiles need to be pried up and a little adhesive removed, while low tiles need to be pried up so additional adhesive can be applied.
Repeat the process until all whole tiles are set into the first section before beginning the next. Do one section at a time.
Allow the adhesive to cure for at least 24 hours before walking on tiles. When the glue has set, mark tiles that need to be cut. Hold a tile in place and use a marking pen to scribe cutting lines. The floor should reach to only within 1/4 to 1/8 inch of the walls to allow for seasonal expansion. Make the cuts with a diamond-bladed wet saw. Be sure to wear safety glasses, as these saws kick out a lot of dust, water and debris. Keep fingers and hands well clear of the blade.
Butter the backs of cut tile pieces before setting them into place on the floor.