To use the Marbling Rake, start by painting the 'raw materials' that will have marbling applied to it. Create some lines of color across your page, using whatever drawing or painting tool you like. The lines here were done using crayons.
Vary the thickness of the colors, and don't worry if some of the white shows (the Marbling Rake will 'distort' the white in the same way as the other colors).
Then select the Marbling Rake tool from the Distortions palette, as shown in this photo.


