Cover the surface you've decided to marbleize with a couple of coats of white paint.
Add a little light gray oil paint to the oil-based glaze. Use a bristled paintbrush to apply a thin layer of paint.
While the paint is still wet, soften the glaze with a lint-free cloth by rubbing the surface gently. This will blend the colors and remove the brush marks. If you are working on a large surface area, you'll want to work in sections so that the paint doesn't dry before you can soften the glaze.
Thin some black paint with some oil-based glaze. Dip a feather into this mixture and gently draw across the surface to look like the veins of the marble you are imitating.
Blur the outlines of the veining with the softening brush. Be sure to wipe this brush regularly on a dry cloth to avoid smudges.
Dip your fine artist's brush in the white spirit and trace over the veins you drew. This will make the lines thinner and soften the effect further.
Repeat steps 4, 5 & 6 to add a second color, if you wish.
Allow the paint to dry per manufacturer's instructions. Protect your marbleized faux finished wall, floor or piece of furniture with several coats of varnish.