Once you get three or more tiles in place next to one another, you can use a 2x4 to ensure that they all have an even height set into the mortar. Place the wood on the floor, stretching across the tiles. If the marble is polished you may want to cover the front of the wood with a piece of carpet to prevent scratches. Just ensure that the surface remains flat and consistent.
Then, lightly tap the middle of the wooden plank with a rubber mallet. As it goes down both sides will also go down, ensuring that all of the tiles have a consistent level. This can be done across multiple rows as you work.