Measure your frame and your art. The size of the art will be the size of the inside edge of the mat, and the size of the frame will be the size of the outside edge of the mat. Note that the measurements of the mat will be about 1/4-inch smaller than your photograph and frame.
Turn the mat over with with back side toward you. If the sides of the mat are not marked or colored differently, you should still be able to discern the back of the mat from the front because the pre-cut slits will be visible on the back.
Select the slits that correspond to the mat size you need. The sizes of the mat after cutting at each score should be clearly marked. If not, simply measure the size of the boxes created by the score lines to select the correct box.
Lay a ruler or other straight edge along the first scored line, and run a hobby knife along the straight edge in the score. This will cut the mat board. Turn the mat, and cut along the next side of the scored box. Continue until the entire mat has been trimmed.
Follow the same procedure for the inside edge of the mat.