The mortar is spread.
How to tile over uneven walls.
The floor slanting is not that much of a problem even for a do it your selfer.
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.
Apply just enough of the adhesive to attach a few tiles at a time so that the adhesive stays moist and ready to receive the tiles.
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.
Mark the wall in the middle with a chalk line to create a guide for the tiles.
With a notched trowel apply the adhesive at the bottom of the cinder block wall.
As for the floor being out i would pick a comfortable height the lower the better and establish a level line and back lay the tile to the floor but i would not do that if you are really talking about true cove tile because you will end up with a cut edge on.
The remaining pieces of the spacers will be hidden by grout.
Hold the float at a 45 degree angle to the.
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.