Place the center tile, using the type of adhesive recommended by the manufacturer. Sometimes this is thin-set mortar but sometimes tile manufacturers recommend a synthetic adhesive. Hold the notched edge of the trowel at about a 45 degree angle to the floor and apply the adhesive to the floor.
Press the tile in place while twisting it back and forth to secure it into position. Pull the tile back up and look to make sure the adhesive is in contact with all of the bottom of the tile. If you still see bare spots on the bottom of the tile you need to make the adhesive thicker when applying it with the notched trowel. Reinstall the center tile.