Comb walls then back butter tiles accordingly if they are really bad holes etc you can build out wall in layers using rapidset adhesive which will set in 3hrs which will then allow you to use your tub stuff after priming make sure you smoothe the rapidset otherwise you are giving yourself more hassle.
How to tile over uneven walls.
The remaining pieces of the spacers will be hidden by grout.
By uneven i think you mean that the walls are not flat.
You can build out locally to a certain extent using cement powder tile adhesive or a quick set render but you should not exceed a depth of 12mm.
Back buttering is a trick used for tiles of uneven thickness like ungauged slate or handmade tiles extremely large tiles or for tiles being installed on an uneven surface.
Mark the wall in the middle with a chalk line to create a guide for the tiles.
Hold the float at a 45 degree angle to the.
The mortar is spread.
With a notched trowel apply the adhesive at the bottom of the cinder block wall.
Don t use plaster or ordinary sand cement render or you will have to wait a minimum of 10 days before tiling longer the thicker it gets.
The floor slanting is not that much of a problem even for a do it your selfer.