Write an article based on this "Measure how big the picture is that you need to mat. Calculate how big the border should be. Mark the back of the mat board so you can line up your cuts. Cut out the center of the mat based on the measurements you made."
To determine where you need to cut your mat, you first need to know how big the picture is. Take your ruler and measure both the width and the height of the photo; write down your measurements on a piece of paper. If you’re framing a photo with a white border, measure from where the actual photo starts so that the white won’t be visible once it’s framed. Now that you know how big the picture is and how big the frame is, you can do some math to determine where you’ll need to cut the mat board. In general, you want anywhere from a 1⁄2 inch (1.3 cm) border to a 2 inches (5.1 cm) border going all the way around the photo. For example:  If your picture is 7 inches (18 cm) wide and 12 inches (30 cm) tall and the frame is 16 inches (41 cm) wide and 20 inches (51 cm) tall, you’ll subtract the picture’s height from the frame’s height and divide that answer by 2 to figure out where you’ll need to make your mat cuts. Similarly, you’ll subtract the picture’s width from the frame’s width and divide that answer by 2 to determine where you need to cut for the width of the mat. If your picture is a lot smaller than the frame, you’ll be left with a much bigger mat border. Always mark the back of the board rather than the front. If you mark on the front, you’ll have to erase your pencil lines which could change the color of the mat or ruin its appearance (especially if it’s not white). Use your metal ruler to mark off the entire square that needs to be removed to create the border for the picture. Use a pencil rather than a marker to make your notations. A marker could bleed into the mat board or could get on your hand and then transfer to the front of the board or picture later on. Line up your metal ruler along one of the lines for the border. Use your X-ACTO knife to carefully cut along the ruler, making sure to start and stop at the intersections with the other lines. If you go too far, you’ll be cutting into the border itself and will need to start over. Cut the board on all 4 sides and then pop out the interior section to reveal the border. You can save that interior section of mat board to use on other projects. You could always make another border for a smaller picture, or use it in other crafting projects.