The paper is white cartridge paper, with thick & watery mix of yellow and ultramarine acrylic paint smeared all over it mixed with water and/or pva glue, then a piece of clingfilm (saron wrap I think it's also called) is laid over it and 'wrinkles' encouraged. It's left to dry but not to dry too much. This is tricky, too long and you end up with the clingfilm attached to the paint and big holes in your paper, too wet and you end up with no wrinkles. Depending on the weather/ warmth of the house, it can take a couple of days to dry enough. Then, leave it for a couple more days before using it, as if the acrylic's thick, you can find it's only dried on top and wet paint oozes out when you cut it.
I think this is my favourite so far, it's going to be hard to give it away!