I fold the fabric into a manageable size; so for a yard, I fold it in half lengthwise on the selvage, then pin the fabric and then line up a ruler up to an inch from the edge of the selvage, I then use a rotary cutter. if the fabric is more than a yard, make sure that the fold is no more than 24" so that it is manageable to use a 24" ruler. Any longer and it's bound to wiggle thus giving an uneven edge.