A photo mat is an essential part of framing a photograph. It sets it out, showing it off separate from the frame. Sure, you can buy them or even have them custom made for your photo project, but why not make your own? Give it a try to see if your results are satisfactory or better. Plain photo mat sheets are inexpensive and you can easily cut it to fit your photo and frame.
Lay your photo mat upside down on a clean surface. You do not want to make any marks on the front side or they will show up clearly when it is under glass. You will also want the surface to be something that you don't mind cutting on, like a piece of cardboard or wood since the razor edge of the mat cutter will score it.
Measure the size of your frame. Mark this on your photo mat, and cut it with a regular utility knife and a steel straight edge. Find the center of the newly cut mat by measuring from side to side and from top to bottom. Divide these both in half and mark where they meet in the middle of the mat.
Measure your photo. Make sure you leave at least a 1/4 inch around the edge so that it sits behind the mat and no edges are showing. Take the side measurement and divide it in half to get your photo center point. Start with the center point on the mat and measure out the half measurement figure you got for your photo. Do the same procedure for the top-to-bottom measurement.
Pencil in the square you wish to cut out using the markings you previously made. Place your steel straight edge along the first line you wish to cut. Set your mat cutter right up against it, and push or pull it slowly but firmly along the edge, starting 1/4 inch before the line and going 1/4 inch beyond it. Alternatively, you can use a utility knife, but the mat cutter holds the blade at a precise angle for a professional edge.
Cut each of the other three sides in the same manner, making sure to cut all the way through. Lift the photo mat sheet up carefully and remove the middle section. Turn it over and inspect for clean edges. The photo mat is now ready for your framing application.