Determine how you want your tile to be laid out, and find a starting point from which you will start your tile laying. Use the notched trowel to spread an even layer of thin-set mortar on the floor in the area where you are starting your 20x20 tiles. Make sure that the notches are of even height across the entire surface where the thin-set mortar has been spread. Use the notched trowel to spread an even layer of thin-set mortar on the back side of the first tile, making even notches across the surface.
Set the tile in place, and press down firmly on the tile. Use the level to ensure that the tile is sitting flat. If the surface of the tile is not level, use your hands to press down on the corners of the tile to adjust the level appropriately. The leveling of the 20x20 tiles is easier because of the larger surface area. You may have a better finished project because of using the 20x20 tiles than you would if you had chosen a smaller tile size. Do not use shims or other objects to adjust the level of the tile because this will cause gaps in the thin-set mortar, ultimately resulting in damage to the floor.
Use the notched trowel to spread an even layer of thin-set mortar onto the floor, ensuring that the notches are even. Use the notched trowel to spread thin-set mortar on the back of one tile at a time, making sure to have even height notches.
Set the tile into place, one tile at a time, next to the existing tile. After setting each tile, place tile spacers around it to create even grout lines. Press firmly down on the tile to ensure that the thin-set mortar on the tile bonds to the thin-set mortar on the floor. Make sure that the edges of the tile match up to that of the existing tiles which border it. Also, make sure the tile is level, and use a straight edge to align the tiles.
Cut tiles to fit the edges of the surface area you are tiling. Measure the space with a tape measure, and mark the corresponding measurement on a tile. Use a wet tile saw to cut the tile along the marked line. When cutting 20x20 tiles, you may have to cut from one side of the tile and then turn the tile around to cut from the other direction depending on the size of your tile saw. Also, when cutting porcelain tiles, you will want to cut through a honing stone first with your tile saw to ensure that the blade of your tile saw is in optimal condition for cutting the porcelain. If you do not do this before cutting the tile, the porcelain may chip along the cut. Set the cut tiles in the same manner as the other tiles.