When my grandaughter started sewing in 4-H, they taught a method of straightening fabric that I had never seen and I have sewed for over 45 years.
Take and pull a thread at both cut ends. Cut along the pulled threads. Fold the fabric in half so the selvages meet. Pin along the cut edges... matching the edgeds.... and along the selvage edges. Don't worry if the fabric is skewed. Machine baste along the 3 sides. Wash the fabric and dry. When the fabric came out of the dryer, it laid flat.