Measure the dimensions of the picture you want to matte. The measurement of the picture is what determines all of the cuts for the matte. Write this number down so you don't forget it.
Subtract 1/4 inch from each side of the picture to determine how large your viewing window will be. The viewing window of the matte should be slightly smaller than the actual picture because you have to attach the picture to the back of the matte. For example, if you have a picture that is 10 inches by 15 inches, subtract 1/4 inch from each side (left, right, top and bottom) to make your viewing window 9.5 inches by 14.5 inches. If you cut the window the same size as the image, the picture would fall through the matte.
Measure the size of your matte board. When you find the width and length of the matte board, subtract the width and length of the viewing window from it. So, if your matte board is 14 inches by 20 inches and your viewing window is 9.5 inches by 14.5 inches, you have 4.5 inches by 5.5 inches of matte board you don't cut. Write these numbers down to help you remember them.
Decide if you want an even board for both the top and bottom and left and right sides. For example, if you want the left and right sides to be the same width around the viewing window and you have 5.5 inches of the width matte board to work with using the measurements above, mark the matte to be cut in half. This will create left and right sides that are 2.75 inches thick around the viewing window.
Mark the top and bottom sides according to the thickness you want them, using the above example. Once you know how thick the borders around the viewing window are going to be, mark them off and cut your window.