first of all is it the floor or the walls? if it's the floor remove tiles and clean and then take your tile thinset and apply to the floor before your hardibacker and adding more thinset to the areas that need leveling and secure with roofing nails. Leave gaps between the haribacker for when you set your tile, because the thinset on top and on bottom of hardi will connect and make one solid surface. You can continue to level with the thinset for the tile...........If you use self-leveling put under hardibacker, but remember to thinset backerboard...you can use a lower grade thinset under board and higer grade under tile.................Eric