Pick the mat color and size ahead of time.
Turn the mat over and find the center of the mat by measuring out from each side into the center.
Once you have found the center, measure out towards one side the length of one side of the picture. Subtract one half of an inch from the line.
Once you have measured out to one side, draw a straight line.
Draw another line to the width of the picture, minus one half an inch, then draw another line.
Repeat steps 3 to 5.
Using an Exacto knife, or utility knife, and a straight edge, cut along each line slowly. Once all lines have been cut through, remove the piece in the center of the mat.
Finally tape the picture in place.