Wash the muslin in a washing machine with laundry detergent.
Remove the wet muslin from the washing machine and move to a waterproof surface. Crumple the fabric into a loose, messy ball using both hands. Wrap twine or large rubber bands around the fabric gently.
Start the washing machine again and fill with enough hot water to hold the muslin. Put on rubber gloves to protect your hands and mix 2 cups of water and the box of RIT dye in a cup until the dye dissolves. Pour the mixture into the washing machine.
Put the muslin into the washing machine and set the machine's washing cycle for 30 minutes or more. Let the machine run the cycle and then rinse in cold water.
Remove the muslin from the machine. Cut and remove the twine or rubber bands. Hang the cloth to dry.
Add 1 cup of chlorine bleach and detergent to your washing machine and run through a full wash cycle to remove any dye residue.