To get an authentic marble look, add veining and cracking with white paint. Mix equal amounts of white paint and thickener/extender, dip a feather (included in the kit) into the paint mixture, and apply veins diagonally at random. It's best to work "nervously," without much control of the feather, to get a realistic look. Add as much or as little veining as desired. Remember, real marble varies.
To add a crack, use the feather and the white paint mixture to apply a line thinner than a vein in the opposite direction as the vein. When intersecting the vein, move the feather down slightly and continue the crack to replicate marble shifting when it cracks in real life. Let the veining dry.
Apply a coat of glossy acrylic spray finish for a high shine.