Alright, so I've gotten dozens of PMs about how to do this, so I figured I would throw together a little tutorial.
What you need:
Pot to dye in
4 plastic grocery bags
about 3-4 ounces of dye (RIT and water)
weight (i used little plant pots)
Get your pot out and put the four grocery bags in there like a clover. Let them puff out a bit before doing step 2.
I put the 3-4 ounces of dye in those little plastic cups that you get with egg dyeing kits, then i microwave it for about 30 seconds so the dye can warm up. Then I drizzle the dye over the bags after they've puffed out a bit.
Put your disc in there face down, masking off anything you DON'T want marbled. Try not to wiggle the disc once it's set down on the dye, or your marbling will not be as clear.
I use two small planters for some plants that never got planted to weight the disc down, so the bags are pressed pretty firmly against the disc. Then comes the hard part...the wait. I've actually only waited like 3 minutes before and got a good marble, but that was because I forgot to mask off the dye I had done on it already, and didn't want the rest to get marbled. It actually came out good, but anywhere from 30 minutes to 12 hours will work pretty good. If you pull it out and it's not as dark as you'd like, throw it back in there.
Or, if you want to add more depth to the marbling, take the disc out, rearrange the bags, and put it back in there to get two different marble patterns. You can also do this with a second color and you get some amazing looking marble.