With any marbble or natural stone you want to be as flat and smooth as possible before you start.As you go along you can make minor adjustments in the heighth of the tile by back buttering the areas that you want to raise up to match the tile previously set.
Use a 1/2 notch trowel and either skim the backs of the tile before you set them with thinset or take a damp sponge and wipe the backs of the tile to get the dust off them so the tile will bond better to the tile.
You may end picking up the tile 2 or 3 times to get it the way you want it with no lippage as it is reffered to,level is not the preffered way anymore as it takes too much time to get it perfectly level so owners of tile co's don't want it level anymore just no lippage is the rule.
Now from a tile mans point of view there is no compromise it is level or nothing,but it depends on you ,what you want to look at,if it were me i would put a level on it.
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