Measure Twice, Cut Once
You have everything ready and are situated at the ideal workspace, but before you start slicing the mat, you need to take proper measurements and make legible markings.
The first thing to measure is the mat to the picture frame. If you purchased a mat the same size as the frame, place it in the frame to make sure it fits properly, trimming any edges as needed. On the other hand, if you are using a full sheet of mat board, measure a section the size of the frame using the straight edge ruler and pencil. Taking these measurements twice is advisable, just in case you measured incorrectly the first time.
When the outer edge of the mat is trimmed to fit the frame, you can prepare to cut out the center. Determine how much space you want to fill between the photo and the frame and then draw a rectangle in the center to fit. For example purposes, we will say that you want the mat to be one-inch wide between the frame and the photo. Use the ruler to measure one inch in from the side of the mat and make a mark. Repeat this measurement in increments around the frame and then use the straight edge to draw a rectangle an inch in from the borders of the mat. Measure a second time, just to be safe.
Cut out the rectangle with your craft knife. Alternatively, you may wish to invest in a mat cutter if you plan to mat and frame a great deal of photos. Place the mat in the frame and then frame the picture. Using acid-free tape to adhere the photo to the mat allows you to frame it more precisely. Replace the backing on the frame and enjoy your custom photo.