Advice for installation of Marble Fireplace Please?

you could use white thin-set that's the same thing as installing tile on the walls do not get the premixed one the one you have to mix with water only home depot should have some and apply that to the back of the marble and push it against the wall you would do the sides first or legs then apply the hearth to the legs with the same mortar or thin set and push every thing together but make sure there are no dust no the marble where the thin set would go let it squeeze from the side it's OK then wipe the excess with a sponge and water good luck