Dyeing in plastic bags, popularized by artist Helen Deighan, is a popular, low-mess method for dyeing small fabric items like scarves or t-shirts, using the bag as a contained dye bath to create unique patterns (mottled, speckled, or all-over color) by manipulating dry or wet fabric with dye, soda ash, and salt solutions, then sealing and agitating for hours to let the dye process. The bag method offers good control for clear, strong colors and is suitable for various effects by changing how the fabric is folded or whether it's wet or dry when added.