I align the 2 selvages edges along my yardage. I fold it again to fit my cutting board. If you have the large cutting mat you don't need to fold the fabric twice. Use a ruler that will go across the fabric you want to cut. I use my large 15 inch square and line one side of the square against the fold. I cut on the opposite side being careful to keep the square steady. After the edge of the fabric is trimmed, I use my square to start cutting the width of fabric I need. I do not use the lines on my matt for cutting.