Thoroughly clean your mirror using a lint-free cloth and alcohol or glass cleaner. This is an important step because dirt or lint on your mirror can cause your glass cutter to skip and scratch the mirror. Protect the backing on the mirror by either applying contact paper to the back or laying a clean towel beneath your mirror, on top of your work surface.
Use the tape measure to determine where you need to mark the mirror for scoring. Use the straight edge ruler as a guide to draw a straight line with the felt marker. You will follow this line to score the mirror with the glass cutter
Roll the glass cutter along the line you drew with the marker. Make sure to hold the glass cutter completely perpendicular to the glass---no leaning---and continuously roll it from one edge to the other. Start at the edge closest to you and roll toward the edge farthest away from you. Roll it in a slow, but continuous motion. You can continue to use the straight edge ruler as a guide to keep your line straight.
When you've completed step three, you should see a very lightly etched line in the mirror. Lay the straight edge ruler and glass cutter aside and lift the mirror tile with both hands. Your right hand will be on the right edge and your left hand should be positioned along the left bottom corner edge of the mirror. Grasp the left bottom edge firmly with your left hand and, applying gentle pressure, make a bending motion. The mirror should snap along the score line.