Marble
The only sure thing about marble is that the is no one way to do it. Every reference of marble has new characteristics that you would approach differently with a new set of tools.
However there are a couple hints I can give you:
For your base coat scumble together 2 to 3 colors
Always take note of patterning and directionality of your sample
Consider using non-traditional tools such as burlap, sponges, feather dusters, sprayers.
Let layers dry, you don’t want to get soupy.
If you have marble with large chunks in it mask off with pieces of bogus paper
You can always go in with a glaze after it’s dry to add contrast.