I first dyed the t-shirt in Gold RIT dye for about 20 minutes. I then rinsed the t-shirt, untied it, re-tied it using the techniques below again, and then dipped it in Purple RIT dye for another 20 minutes. The picture here does not do the t-shirt justice, I should have repeated this process a few more times to get a better marbled effect.
For more tips and dying directions, check out this tutorial.