Dezeray S. provided a link that gives some good tips on washing burlap. Here is some additional help:
It may be difficult to make the burlap soft. You can clean it and get most of the smell out by hand washing it. I don't recommend that you use a washing machine to wash the burlap. Since you are making curtains you may need a large sink or the bath tub to wash the burlap. Fill the container with lukewarm water. Add a small amount of a mild liquid detergent or liquid laundry detergent. Immerse the burlap in the liquid and let it soak for about 5 minutes. You may need to continue pushing the burlap down into the water as it will tend to float. Pull the burlap out and drain the soapy water. Rinse the container and then put the burlap back in and rinse it thoroughly. Lay the burlap out on towels and let it air dry. This should remove most of the smell. Repeat this process if necessary to remove more smell. Washing the burlap may take out some of the stiffness but not much.
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