Fold the shower curtain in half grommet to grommet. Make sure the folded width will easily fit in the plastic container. Knot a piece of twine through the grommets at the top of the shower curtain. This will ensure that the fabric stays even from the folding and will help maintain more even lines with the dye.
Begin lowering the bottom of the shower curtain into the dye bath. Lower it down to the highest point you want dye to appear on the curtain. Our shower curtain was left white at the top, so it was dipped into the dye bath about 12 inches from the top.
To achieve a very light color at the top, leave the top portion in for just a few seconds or until desired color is achieved. Then pull the top portion out of the dye bath and rest it on a tall standing object or hold it up on your own. Continue to do this for the entire curtain in 1- to 2-foot increments, lifting a new section up and out of the dye bath every 1 to 3 minutes to achieve the light-to-dark effect.
Wring out excess dye from the shower curtain and remove it from dye bath. Carefully bring shower curtain to a sink and run cold water over the entire dyed surface. Hold the lightest color closest to the water to ensure that the dye flows down from lightest to darkest. Continue until water runs clear. Wring out excess water. Launder the shower curtain according to instructions on the bottle of dye.