Floor tile is normally laid out in a grid pattern, by dividing the floor into four quadrants and then tiling along the lines from the centre outward to the walls. However, another option is to tile in a diamond pattern. This is just a standard grid that's been turned 45 degrees, so it's diagonal to the walls. This is a good solution for a room that's badly out of square, a problem that a square grid will highlight but a diamond pattern won't. Start with a solid, firm underlayment.
Measure all four edges of the floor and find and mark the middle of each side. Stretch your chalk snap line between two of the lines that are opposite each other, and snap the string to leave a chalk line there.
Stretch the chalk snap line between the other two marks that are opposite each other so it intersects the first line at the centre of the floor. Adjust the position of the string to be exactly perpendicular to the first line, by laying a square at the intersection. Snap the string for the second line. The floor should now be divided into four quadrants with a large "plus" sign.
Measure from the centre of the intersection out along each line and mark each one at 3 feet. Connect each of the marks by lines, using a level and pencil. You should end up with the shape of a tilted square superimposed over the intersection of the two original lines, with the corners of the square lined up with the lines.
Measure each side of the square, find the middle and mark it.
Stretch your chalk snap line diagonally over the intersection, so it runs through the two marks that are located at the middle of the two opposing sides of the square. Snap the line.
Snap a second diagonal line in the other diagonal direction, checking the line with your square against the first diagonal line to make sure they're perpendicular. You should have two intersecting lines forming an "X" through the middle of the "plus" sign. Tile the floor using square tiles, installing them along the lines of the "X," starting at the centre and working your way out.
It is a good idea to "dry lay" your first row of tile to see if you might avoid awkard cuts by moving your centre point slightly.