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Soak your sweatshirt in warm water and wring it out. Mix your dye and add it to a large pot or plastic bin. Fold, twist, or tie your sweatshirt into a pattern. Dip the sweatshirt into the dye mixture. Place your sweatshirt in a plastic bag and allow it to soak overnight. Rinse your sweatshirt in cool water, then remove the rubber bands.
Wet fibers absorb dye more easily than dry ones. Make sure your sweatshirt is thoroughly soaked, then wring out any excess water. You can also use this step to wash your sweatshirt to make sure there aren't any fixatives, dyes, or fabric softener residues in the material that will affect color saturation. Follow the directions on the dye package to determine how much water you should add to the dye. Add about a cup of salt to the dye bath to help the color set in. Experiment with different ways of manipulating the fabric for different tie-dye effects. Use rubber bands to tie off sections of your sweatshirt, making sure to secure it tightly so the dye doesn't bleed.  You can get different looks by folding, swirling, and crinkling your shirt. Use multiple rubber bands in the same area to get a bulls-eye effect. Your sweatshirt should be immersed in the dye for about 10-30 minutes, depending in the color you're trying to achieve. Leave it in less time for a lighter color or more time for a deeper shade. If you're dying your shirt more than one color, you'll need to hold each section in the dye for the required amount of time. This takes a little patience, but it really allows the dye to spread into the folds of fabric and create those cool tie-dye effects. After allowing your fabric to rest overnight, rinse the dye out with cool water until the water runs clear. After it is thoroughly rinsed, cut off the rubber bands and see your new masterpiece!