Snap a diagonal line over the center of the floor, from corner to corner, using the chalk snapline.
Pull the snapline between the two remaining corners, intersecting the first line at the middle of the floor. Put a carpenter's square at the intersection to make sure the lines are perpendicular to each other. Move the string if necessary, and snap the line. This should leave an "X" over the middle of the floor.
Spread thin set over the center of the floor, covering four or five square feet within one of the four corners formed by the two lines.
Press a tile into the thin set in one of the four corners formed by the intersection. Press additional tiles next to it, setting spacers between them.
Press more tiles in place, working your way out from the middle toward the edges, using the lines to keep it straight. Lay down more mortar as needed.
Lay all the full tiles that will fit. Cut tiles for the edges of the floor on the tile cutter, laying them with the cut sides facing the walls.
Allow the tiles to set for 12 hours. Pull out the spacers.
Lay grout over the floor with a grout float, forcing it into the gaps between the tiles. Wipe up the excess grout with a damp sponge.