The marbling process can be messy and involves indelible sumi, so plan accordingly. If you don't want to risk your indoor work space consider try it in a relatively safe environment--an outdoor patio or some such location.
Here is a list of what is needed:
Paper (lots) for marbling and subsequent painting (e.g. rice paper)
(Heaver weight paper works best because light-weight washi can sometimes tear just from its own weight when wet.)Sumi, either liquid or suzuri-made
Liquid dish washing soap
Tub, large enough for the paper to be marbled to lie flat
Two large bamboo painting brushes
Wind source (optional)--natural wind, a fan, or lung power; perhaps through a straw
(A stirring stick--something skinny like a skewer--might be used instead for helping to create sumi patterns on the water's surface.)Two small containers, roughly cup-sized
A brush-holding container
Paper towels (lots)
Water
Drying surface (I use Plexiglass)
Spread plastic sheeting or something protective around your work space to protect from splashes of indelible sumi ink. Accidents aren't inevitable, but it is wise to exercise precaution. Here are other things that need to be done to get ready for suminagashi.
Fill the tub with about 1 1/2 inches of water.
Put sumi in one of the small containers.
Put water mixed with a few drops of liquid dish soap in the other small container.
Have your brushes at hand in the brush-holding container.
Set a pile of rice paper nearby, ready to be marbled.
Have plenty of paper towels ready to absorb any excess water after marbling.
Have a drying surface ready nearby.
Your wind-source and/or skewer need to be ready for use nearby, too.
Here is what you do:
Load sumi on one of the brushes and the water/liquid soap solution on the other.
Holding one brush in each hand, touch the surface of the water in the tub with the sumi brush using just the tip of the bristles.
Then in the middle of the resulting spreading circle of ink, quickly touch the surface of the water with the tip of the other brush.
Repeat steps 2 and 3 in rapid succession. The water surface spot chosen for touching the brushes can be the same or different depending what sort of pattern you want to produce.
You can get thick lines or thin lines depending on how much sumi is transferred from the brush to the water. Experiment to get a feel for how this works.
When you are ready, place a sheet of washi on the water's surface to capture the image. Take care to avoid trapping air bubbles.
Quickly lift the paper and place it flat on your drying surface.
Immediately dab off excess moisture with a paper towel.
About 5 marbled images can be made before needing to change the water and start over.
By judicious use of wind power and/or the stirring stick, you may be able to influence the appearance of the marbled images.
Most of the time, you want to let marbled papers thoroughly dry before using. However if you want ink to blur a little when you paint, slightly damp paper could be useful.
Think about the painting subject you plan to use with marbled papers. You want to avoid marbling and paintings that clash with each other.