What Do I Need? Something to marble, paint, Alum, Carrageenan, Methocel, a large, flat, shallow tray to lay your fabric or shirt in, and combs, straws, sticks, toothpicks, etc. to swirl colors around with.
How much do I need? Well, that's a little bit more complicated, but not too much. To do fabric that's 36" x 36" you'll need a frame 38" x 38" filled about 2" deep which requires about 7 gallons of size. To do fabric that's 18" x 72" you'll need a frame 20" x 74" also using 7 Gal. of size. A roasting pan filled 2" deep uses less than a gallon. Remember you'll have to change the size periodically.
Use about 1/8 cup of Alum per quart of water to pre-soak for fabric. Estimate how many quarts you need to soak your items and this will tell you how much Alum you need. Careful! Too much or too long rots the fabric!
The paints can be thinned with water to help them float. A drop or two of dispersant also helps the paints to float better.