I just got done washing up some red fabric that was so stiff, I didn't think I would get it softened up, but I did.
If you can, hand wash the fabric in hot water, put laundry detergent in the water, splash it until it gets some good suds, then dunk the fabric in and let it sit and soak for about thirty minutes. Give it a good rinse, then soak it again in hot water and laundry detergent, give it a good rinse and then put it in a final bath of warm water and fabric softener and let it soak for at least an hour. Wring it out and hang it up to dry.
I only suggest hand washing because when I prewash fabric, I always do it by hand so I can actually monitor for color bleeding. If you're more comfortable using the washing machine, by all means, do it, just kill the cycle when the tub is about half full and let it soak.