I make my first cut with the fabric folded only in half, with at least a 6 X 24 ruler lined up with the fold on the fabric so that anything on the right gets cut off. (I'm right handed.) Then I fold that in half again and turn it around so that the cut edge is on the left. Now I lay my ruler on the fabric the width I want to cut it. (Be sure to look up and down the entire length.)
Now I place my cutter about two to three inches above the bottom edge and cut back toward myself. This keeps that edge from getting distorted. Then continue the cut, moving the other hand as necessary with the cutting wheel held in place while you move.