Marble typically has very tight joints, no bigger than 1/8th inch. This means it is critical to get the tiles level and even. You can use a mud set technique or a good quality thinset mortar. You will need to use a two foot and a four foot or larger level. Go slow, and remember the key is flat and level with no lippage. You may want to look into using Lash leveling wedges and clips which is supposed to virtually eliminate lippage. Good luck.
