Wash your slipcover in the washing machine before dying to remove any spots and chemicals. Cleaning the slipcover will make the fabric more receptive to the dye.
Purchase high-quality powder or dye for use in a washing machine from a supermarket or hardware store.
Pre-dissolve powder or liquid dye in 3 cups of hot water. Add the dissolved dye to the washing machine.
Fill up a sink with hot water and thoroughly saturate your slipcover in it. Smooth the wet slipcover out on a table. Set the washing machine for the longest cycle so your slipcover will sit in the dye bath for 30 minutes before the rinse cycle. Submerge your slipcover in the washing machine until it's completely covered by the dye bath.
Rinse your slipcover in one or two cold-water rinse cycles and again in a sink until the water runs clear and there are no traces of dye residue. If you used a dark-colored dye, run the fabric through another wash cycle in warm water using a mild laundry detergent. Dry your slipcover in a clothes dryer or air-dry on a clothesline.