If you have a concrete floor and you want to lay ceramic tile, vinyl, carpet or even wood, it's important that the concrete be solid and flat. If the floor is wavy, cracked or otherwise compromised and is beyond what you could reasonably fix with a liquid floor leveller, then another solution is to lay hardboard over it. Hardboard, also called cement board, looks and cuts like drywall, but once installed, it has the flat solidity of a layer of cement. The existing concrete floor needs to be completely free of all oils and other impurities before you lay the hardboard, since you can't screw it down and must rely entirely on the mortar bond.
Combine ½ cup TSP with 1 gallon of hot water. Scrub the cement floor thoroughly with the solution. Rinse well. Let the cement dry for 48 hours.
Set a piece of hardboard in one corner of the room, on the floor and bordered by two walls. Trace around the edge of the board with a pencil. Remove the board.
Spread thin-set mortar over the marked area, using a notched trowel.
Press the hardboard back in place. Apply your weight over the whole board, walking on it to press it completely down.
Lay additional sheets of hardboard in the same way, arranging them end to end along one side of the room. Cut the final board to fit by scoring it with a utility knife and bending it until it breaks at the scored line.
Set the second course of boards starting at the opposite end that you started from before, so the ends of the boards don't line up between courses. Cover the entire floor. Let the mortar set for 12 hours.
Lay drywall self-adhesive mesh tape along the seams of the hardboards. Spread thin-set mortar over the tape, smoothing it out with a drywall knife. Let it set for 12 hours. The surface is now ready for floor covering.