Iron the whole piece flat first. Then grab the two selvages together and slip them back and forth until the bottom of the fabric (where your fold will be) is hanging straight. When you start slipping the fabric left and right you'll see when it has a twist and when it's straight pretty fast! Carefully lay the fabric onto the ironing board and finish your ironing if you want to do more. Remember, don't use stem on your fold or the crease may become permanent!
Often times after I've got the fabric straight I'll just lay it on the cutting board and hand press it. Then fold it over one more time and straighten the end of the fabric.
I hope this helps.