Read a thorough breakdown regarding Can I Use Vinegar to Set Rit Dye.
Powder dye should be pre-dissolved in 2 cups 470 mL of hot water and then added to machine. Mix well to evenly distribute the dye. Wet fabric and place item(s) in the washing machine. If dyeing natural fibers like cotton or linen, add 1 cup 240 mL of salt to the dyebath.