I do what Tartan does. No tugging and pulling because that just distorts the fabric more usually. I align the selvage edges as best as I can. If it is a smaller piece...say a yard or so...I will try sometimes located the mid point of both ends between the selvages and let it fall. Hopefully this will get the selvage edges close to each other but sometimes it doesn't. any way after aligning the selvage edges, I most often find that the ends of the folded fabric are not even. So then I take my ruler, aligning one of the lines on my ruler with the folded side on top and cut off that crooked edge so both sides are even on the cut edge. The crooked cut off piece goes into my scrap bin if it is wide enough for a string. When I am cutting by ruler, often I find after making several strips, I have to recut off a bit is the edges are no longer straight. I have had some yardage that is so off on the bolt that the selvages are no where each other. Sometimes I just throw that piece into the washer and dryer, press it, and fold it so the selvages meet. It seems easier to get it straightened out after the washing and drying.