Protect your work surface with a flat piece of cardboard, an dthen place your mat board on top. Place your picture frame against the backside of your mat board and trace with a pencil to get the outside measurements for the mat.
Practice cutting the mat board with the craft knife outside of the traced area to get a feel for how to make a clean cut. Using a sharp craft knife, press down firmly and move the craft knife down the board in a straight continuous line. Once you are ready, place your ruler on the pencil line to use as a guide and run your craft knife down the line in one continuous motion. If it does not cut all the way through the first time, simply repeat.
Place your photo or artwork on the mat and decide how much of the photo or artwork you want to see through the mat window. The mat must cover at least a 1/4-inch border all the way around the photo or artwork to hold it in place, but you may want your mat to cover more to draw attention to one part of the photo or artwork.
Measure the part of the artwork or photo that you want visible to get the measurements for your mat window. Using your ruler and pencil, draw the outline of the mat window on the backside of the mat. You can center the mat window for a traditional look or place the window slightly higher, lower or otherwise off-center to create a personal look.
Cut along the lines of the mat window using your ruler as a straight edge. Cut in one continuous line, stopping quickly when your reach a corner. If the corners do not cut cleanly on your first try, do not pull at the excess mat in the corners, as this can cause unsightly tears. Instead, go back over the corner slowly, tipping the blade of the craft knife down and applying pressure until the cut is clean.