Flat floor, isolation membrane (Ditra is my standard), a more deeply notched trowel (1/4" wide x 3/8 deep at least) so that you don't have areas without thinset adhesion and so that you can adjust levelness. One of the tile leveling systems is helpful with large tiles (I personally don't like the QEP LASH system (the one carried by the big boxes), but you can find threads on the subject). Go through all the tiles and find the ones that are most-bowed - even the most expensive tiles will have some bow. If you don't have enough tiles to do without them, use the bowed ones for cuts.
It wasn't your question, but adjust your thinking about ordering tiles and managing or planning for breakage or waste. With big tiles in small spaces, you can't just measure the square feet and add 5% for breakage and waste. The layout and the quality of the tile (will it break when you cut it) can have a big effect on how much tile you need.