Measure the piece you are cutting the mat for, then lightly draw these dimensions on the back of the mat board.
Plan the shape you want your "window" to be, then trace or sketch the shape just inside the edges of the photo or article's dimensions. In order for your mat to frame your piece properly there needs to be at least a quarter-inch overhang. Measure at least a quarter of an inch closer to the center of the window, then retrace the shape keeping the distance the same throughout.
Use the mat cutting knife to cut out the final shape of your window using smooth strokes.
Use the razor blade or utility knife to shave away any leftover fibers on the corners of the mat board.