Prepare the subfloor by using an all-purpose cleaner and cleaning rags. Rinse and dry the subfloor thoroughly. Allow it to air-dry for at least six hours.
Fill in or smooth out any uneven areas on the subfloor with patching material and a trowel. Allow the patching material to dry.
Cover the subfloor with the roofing felt to provide a moisture barrier between the floor and subfloor. Prevent the roofing felt from moving during the underlay process by stapling it in place with a construction or industrial stapler.
Place 2.5-gauge wire mesh over the roofing felt. Overlap each section of wire mesh by 1 inch. Secure the wire mesh into position by stapling it.
Pour ceramic floor mortar mix into a large utility bucket. Add enough water to make the mud mix moist but not wet.
Put on rubber-coated work gloves, and test the consistency of the mud by squeezing a fistful into a ball. If the mud holds its shape, it's ready to use.
Start pouring the mud over the wire mesh at the furthest point away from the door. Pack the mud down using the board, pushing it in a downward motion as you work your way toward the door.
Place the 3-foot level on the mud underlay to ensure it's level. Check the thickness of the mud by inserting the end of your tape measure into the mud to see if you're at the recommended thickness of at least 3/4-inch.
Allow the mud bed to dry for 24 hours before installing your ceramic tile.