How to prepare window stool for tiling in shower
As far as getting the existing material any farther back, don't sweat it. The part that will need the attention is the vinyl face of the window frame. Build up the framing with a 5-10 degree slope to the inside of the shower on the bottom  and square with the other 3 sides. Set them so the framing is even with the edge of the window frame. then add the cement backer. Tape all the joints with fiberglass tape and modified thinset, keep it neat. After it has set for a day, paint it all with liquid applied membrane like Redgard(r) or similar. make a nice line onto the face of the window, high enough to seal, but low enough for the tile to conceal the membrane, let it cure, then set your tile.