Marble flooring is more difficult to install than ceramic, for a few reasons. Marble is a natural stone tile and so is straight-cut along the edges (not mounded or curved like ceramic usually is), so the tops of the marble tiles have to be completely even with each other or you'll have sharp edges on the floor. Marble can't be cut with a standard tile cutter (you need a wet saw), and the tiles should be set very close together to give the impression of a slab. Start with a firm underlayment of cement board or other moisture-reisistant surface for your bathroom floor.

1
Pull the toilet and any other removal obstructions. Remove the floor trim.

2
Stretch and snap a chalk snap line across the middle of the floor, dividing the room in two. Stretch the line in the opposite direction, intersecting the middle of the first line. Square the string to the first line using a carpenter's square, and then snap the line. The two lines should cut the room into four even squares.

3
Spread thin-set mortar over the intersection, for an area as big as you can comfortably reach. Press the first four marble tiles into place at the intersection, putting 1/16-inch spacers between them.

4
Add more mortar and more tiles, building toward the walls along the lines. Stop after every four to six tiles and press the narrow edge of a level over the surface to get the tops of the tiles even with each other. Use a wet saw to cut the tiles to size by the walls, and around the toilet drain and other obstructions. Let them set overnight.

5
Pull out the spacers. Apply unsanded grout over the surface with a grout squeegee. Use the squeegee to press the grout into the thin lines between the tiles while scraping it off the surface. Wipe off the excess grout with a damp sponge.