When you purchase fabric it is never straight. When you put the two selvege edges together you may see the there is a lump at the point of the fold. Move over one selvege edge to the left or right until you get a smooth fabric. Doing this while standing up or else you will not see the problem or the difference in how the fabric lays. Now all you have to do is to use either folded or selvege edge as a straight edge and cut a straigh line from fold to selvege. Cut a small sample to check the straightness of strip. That's it.