The mat board is quite large when you get it at the store. Sometimes you can purchase the "holes" of mats that were done at the store. The corners are almost always crushed to some degree. This mat board was OK, but I thought it would be a good idea to show how to deal with the bad corner. Keep in mind that we are going to cut the mat to finished size, that is 11x14" for an 11x14" frame. If your mount is already cut to size, make the mat a little larger so that the window can be aligned with the print, and the mat trimmed to fit.
I have placed the cardboard backer from the on the mat board. I placed it about 1/2" from each side to stay away from the crushed corner. I make several pencil marks to show the size of the backer. This will be an 11X14" mat. Remember, always work from the back of the mat. This protects the front and allows you to make whatever marks and calculations without messing up the front of the board.
Using the straight edge, I connect the marks to make the two lines for the edge of the mat.
The mat cutter is the easiest thing to use to cut the mat. Here I have set the cutter to 90 degrees and just deep enough to go through the mat and not much further (about 1/32").
With my mat cutter, the cut line (at 90 degrees) is 1/2" to the right of the straightedge. Align the tip of the blade with the cut line and plunge it into the work. Move the straight edge up against the cutter. Then measure the distance from the straightedge to the cut line near the cutter and at the end of the cut. Adjust the straightedge so it is parallel to the cut line. Be sure to use a straight edge with a cork or anti-slip back.
Push down and straight ahead on the cutter. This also puts some force against the straightedge. If you are worried about the straightedge slipping, try some masking tape to hold it in position.
After both lines are cut, the mat should drop free.