Step 1
Measure the floor along each wall, finding and marking the middle of the length.
Step 2
With an assistant, stretch the chalk snap line across the room, from the mark along one wall to the mark along the opposite wall. Pull the string taut, then snap the line (lift it in the middle and let it snap back down onto the floor, leaving a straight line of chalk).
Step 3
Stretch the snap line across the room from the two walls that are perpendicular to the first line. Don't snap it yet, but hook the string to the edge of the floor (or to a partly-set nail) on one side of the room, while your assistant stretches the string taut on the other side.
Step 4
Lay your carpenter's square at the intersection formed by the string and the chalk line that you snapped earlier. If the two perpendicular spans of the square don’t line up exactly with both the string and the chalk line, have your assistant move the string one way or another until they do.
Step 5
Snap the line. Start tiling at the intersection of the two chalk lines, working your way toward the walls.