Use a highly accurate level to transfer perfectly plumb lines from the ceiling down the wall. Then use a highly accurate level to make perfectly level lines on the walls so that your tiles are both level and plumb as you install.
It's been my experience that a majority of levels are not accurate. They get much better as you spend more money for these tools. But you can easily test any given level by reading where the bubble falls within the vial markers, then rotating the level end for end (with same surface of the level against the exact same spot on the wall) and compare where the bubble falls within the vial markers. If the bubble reads the exact same in both positions, it's accurate on that particular vial. Don't forget to check all the vials on the level before assuming that they are all accurate. I check every level I intend on purchasing, from cheap to expensive, before purchasing.