Fill your washing machine with hot water. Set it to the largest load setting to ensure the fabric can move around freely inside the machine.
Add fabric dye to the water according to the package instructions. Different types of dye will require different ratios of dye to water. Also adjust the amount of dye according to how dark or light a shade you desire. The package instructions on the dye will give you instructions on how to reach the desired shade.
Wet the curtains thoroughly with hot water. Wring them out lightly and smooth them out to remove wrinkles.
Submerge the curtains into the dye bath in the washing machine. Set the washer to the longest wash cycle. Turn on the machine and let the curtains run through a complete cycle.
Keep the curtains wet in the dye bath for at least 30 minutes before the rinse cycle begins. If the wash cycle stops before 30 minutes, reset the machine to start the wash cycle again. Once at least 30 minutes has elapsed, forward the machine to the rinse cycle.
Run the curtains through a full cold water rinse cycle. More than one cycle may be necessary. Continue rinsing until the rinse water in the washer is clear.
Remove the curtains from the washer. Use your hands to smooth out the wrinkles and hang them to dry.