Decide on the size of your mat board. If you have an 8-by-10 photo, you may choose to have an 11-by-14 or 16-by-20 mat. Make sure the mat still fills a standard frame size.
Measure out and mark the mat board using the T-square or ruler.
Place the razor blade along the T-square and trim along the edges you've marked.
Measure out equal margins all around your piece. For example, if you have an 11-by-14 mat with an 8-by-10 picture, you may want a 2 3/4 inch margin all around.
Make sure to leave 1/4 of an inch all around the picture to tape your picture to.
Mark these margins along your T-square.
Cut along the markings using the razor blade, making sure to turn carefully at each corner and leave a sharp point.
Pull the center part out.
Tape the picture, face down, on all four sides to the mat.
Place the picture in a frame.