For my suminagashi (floating ink) marbling I’m using a black yuenboku sumi ink from Yasutomo (yuenboku is an ink made from lamp soot). The paper is a “student quality” hosho paper, also from Yasutomo.
And then I use a tray, some water, dish soap, and two brushes. The procedure is simple:
Fill the tray with some water. Mix a little bit of dish soap with water in one cup. Fill ink in another cup. Now, wet one brush with the soap water and wet the other with the ink. Next you’ll carefully dip the ink brush and the soap brush intermittently in the water tray, gently touching the surface with the brush tips. You’ll see circles forming in the water. Continue until you like what you have. You can additionally manipulate the pattern by carefully fanning the water, blowing on it, pulling out a strand of hair from your head (if you have hair) and dragging it through the ink, or whatever you come up with. Now carefully lay the paper down in the water, then lift it out. If the paper is the right type, it will soak up the ink and create an imprint of what you saw in the water. Lay out to dry.