I would first wash and press the fabric (or maybe just press, if the rest of your fabric isn't pre-washed); fabric pressed with Best Press (or starch) is more stable and manageable. Then I would straighten the cut edges in the usual way when preparing for cutting (fold, matching selvages, cut with ruler and rotary cutter). Then I would re-fold the other way and cut off the selvages (as when cutting fabric length-wise for borders). You should then have a straight piece of fabric to work with. I would then lay it out on the floor, determine its measurements, calculate the amount you need to cut off, and measure in that amount from the edges, marking the distance along the length and width. Then I would take it back to the cutting table, re-fold and use my ruler and rotary cutter to cut along the marks; then repeat for the second edge.