Always prewash your items. Then start with a wet shirt (or article of clothing), then tie it, then soak it in soda ash before dyeing. Use dye with water mixed with urea.
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Pinch your shirt where you want the center of the spiral and turn. Then secure it with rubber bands. You can use two or more colors!
You can also increase the complexity of the spiral by using different colors on each side of the folded shirt: http://tie-dyewiki.com/wikinew/index.php?title=Spiral
See also Double Spiral
combination of swirl and a scrunch
Here's where I added stars to mine with a bleach pen:
Do NOT remove the rubber bands or string until AFTER you've put the garment through the rinse in the washing machine. When you're all done and the shirt is dry, iron with a hot IRON.