Flooring Tile - Marble over tile on wood subfloor

Want to lay marble on bathroom floor. Currently have 4x4 white ceramic tiles over a wood 2nd story subfloor. What is my best plan of action to ensure

1) subfloor is flat and will adhese to marble.

2) best way seal (before or after grout)

3) best way to maintain?

Are you saying that the current tile is adhered right to the plywood? If thats the case you should rip it up. If you are saying that there is two layers of plywood and a cement board, or a mud job (1 1/4" minimum mud) over the subfloor than you may be ok. Because you are installing marble your floor has to be twice as strong as it does for ceramic tile. You need to meet deflection for L720 and you have to have two layers of plywood totalling 1 1/4" in total thickness. The more I go on here the more I think you may be better off ripping out what you have and starting over from scratch.

If you have all that, you can tile over a tiled floor with marble. The existing tile has to be in good condition and well adhered to the underlayment. There should be no cracked tiles, or cracked or loose grout. If there is you should rip it all out. The existing tile needs to be free of greese, oils, wax etc. Rough up the existing tile surface with sandpaper to make it a better surface for thinset to adhere to. Use a very good quality white modified thinset.

Other considerations will be that you will have a height issue with the toilet that has to be dealt with. The toilet needs to be removed and your new tile needs to go up to the closet flange. You'll need to use a thicker wax ring and or flange extender and longer bolts to reset the toilet. Also consider height transitions to the abuting room or hallway. You may want to remove the vanity and tile under it as well. This way if you decide to replace it in the future there will be no tile issues.