Cutting Manually
The most basic way to cut mat board is with a metal ruler and a utility knife. This method produces straight-cut edges. For example, if you wanted to make a matted picture in an 8-by-10 size, measure those dimensions onto a full 32-by-40 sheet of matting. Use your metal ruler and a pencil to mark off the lines. Then, measure the center or "window" area where your picture goes. Do this by measuring the dimensions of your picture and then cut the "window" a quarter of an inch shorter on all sides. So, if you have a 4-by-6 photo, make your center cut 3 3/4-by-5 3/4. This way the edges of your picture will not show through. Then, put the mat board on a hard, flat surface, place your ruler along the pencil guides, press down firmly and follow the lines with your knife.
Cutting By Machine
There are machines designed to give you faster, smoother results. The simplest machines are handheld versions like the Dexter mat cutter that holds the blade at an angle for a beveled cut. You still use a metal ruler as a guide as you move the cutting tool across the mat board, but you can now cut in one smooth stroke as you pull the blade along your guidelines. A tabletop mat cutter is the next step up in cutting machines. It has a steel bar that holds the mat board in place and a sliding blade. Tabletop machines feature adjustable stops so that you can lock in dimensions and make the same cut repeatedly, without having to remeasure. Depending on the machine's capabilities, you may even be able to make decorative cuts such as a "V-groove."
Mat Enhancements
Once you cut your mat, you have the choice to use it as is or add enhancements, such as lines and colors around the artwork, called "French lines." You may want to add a simple accent by drawing one line around the picture in a colored pencil. Use a metal ruler to draw the line an inch or two from the picture to create a frame-within-a-frame look. Or you may draw two lines a half-inch apart and then color in the area between them with a pastel. Another enhancement is painting the beveled edges of matting. Angled cuts cause the white color in the center of the mat board to show. You can paint this edge with a color that accents your artwork.