Replace the razor blade in the mat cutter. If the cutter is leaving dull corners and edges on the mat board it is due to a dull blade. Twist off the handle of the cutter and slide out the current blade. Insert the new blade and put on the handle.
Check the screws on the cutter. If the handle is lose the blade is going to wobble, leaving you with crooked mat cuts. Tighten the exposed screw on the cutter to secure the razor blade using a screwdriver.
Adjust cutter alignment on the mat cutter board. A small groove on the edge of the cutter fits onto an extended indentation on the cutter board. This ensures the cutter locks into place, preventing slipping and cutting an incorrect shape. Push the cutting edge flush against the cutter board to ensure it slides into the groove.
Rotate the mat board after you cut one side. For example, after cutting the right side of the mat board, rotate the board 90 degrees and cut the other side of the board. Do not rotate the cutter. The blade on the C&H cutter is at an angle; if you rotate the cutter it will cut a different angle into the board. This gives each side of the cut window a different look, pulling attention away from the item being matted.