if you are talking about 'ipoh white marble', i advise against using it on the floor as it is not as dense and is more prone to chips & cracks. this marble is more suitably used for furniture tops, shelving and wall cladding.

waterproofing of floor slab and wall are to be done before laying marble. usually cementitious would be used for ground floor. if it is for a wet room on second floor, a bathroom/toilet for example, torched on membrane would be much safer.

for 'waterproofing' of the marble on floor, it should be done on every piece prior to its installation with impregnator, whereas for wall installation, it could be done after installation only on the exposed surface to save cost. impregnator usually last for >5years for residential usage