I use marbled paper as my writing paper (yes, I still write real letters on paper and everything!) and I find the best way to do it is to get real paper marbling supplies from the art store.
Then, once you have set everything up and mixed up a bunch of colors you like on the water, you dip as many papers in as you can, as carefully as you can.
While you won't get the same pattern each time, and that's not the point anyway, you will end up with a matching set of colors. That way, you can make sure your letters won't be complete eye sores of clashing colors.