Or, if you want a realllllly tedious method - align as AndiR says, getting your single fold straight to one side and/or top/bottom edges of the total piece. Then, find your widest edge that is off, and bit by bit, pull the short threads from that 'un square' corner until you get to a single thread the length/width of your entire piece. That will give you a truly straight-of-grain edge to then work from.