I would suggest fold in half - selvage to selvage. Smooth from the folded edge to the selvage. While smoothing to the top edge, smooth to the outside edges as well. You should be able to get both sides (top & bottom that is) simultaneously. Your side edges WILL NOT match up. This is where you are going to take your ruler and line up the narrowest point to chop off to make that side square. Then you will get a straight edge from which to start cutting. You will probably need to flip to start cutting. I try, if I have enough room, to move my body to the opposite side of the cutting table instead of moving the fabric. It stays more stable that way. You may need to make small 'adjustment' cuts to straighten along the way but they will be minimal. Hope this makes sense and works for you.