Hello the first thing you want to do is determine whether or not the exsisting floor is solid with no humps or rotten-damage to it.If it is acceptable then the next thing would be choosing the thickness of hardibacker.Hardibacker is a concrete type substrate installed to the exsisting floor prior to tile.It comes in 1/4"and 1/2" thickness.If u are connecting to other floors in the home this is where you would choose the thickness of hardibacker so all floors meet without transition.Once you choose the size the hardibacker is then installed by mixing up full flex, applying it