Measure out the size of the picture you want to mat. Now decide how large you want the mat around the picture to be. You may want a 2-inch border around the left, top and right side and a 3-inch border around the bottom.
Add up your sizes to determine how large a piece of a mat board you need to purchase. If your picture is 18 inches high and 24 inches long and you want a 2-inch border on the top of the picture and a 3-inch border on the bottom, you would add 5 inches to the height of your picture to calculate mat height (23 inches in this example). Do the same with the width. If you want 2-inch borders on both sides, add 4 inches, giving you a final size of 23 x 28.
However, you're not done there. You need to leave a slight overhang so the picture can be attached to the mat once it is cut. A good size for this is 1/2 inch for each side. 1/2 on each side equates to 1 inch for both the height and width. Reduce the overall mat size by 1 inch per side to account for this, giving you your mat size of 22 x 27.
Clean off a table and place the mat board face down onto it. Take out your ruler and pencil and draw two inches from the edge on both the top, left and right sides. On the bottom, draw a line three inches from the bottom. These lines are going to help you when cutting out the window for your mat.
Align your mat cutter ruler along the first line you drew (it doesn't matter which line you start on). Place the matter cutter's blade onto the intersection of two of the lines you drew out (this is a corner of your window). Press the blade firmly down into the mat and hold the mat board strongly while you glide the blade along the ruler until you reach the drawn line intersection at the other end of the board.
Repeat this cutting process until all four sides of the window have been cut and the middle section of the mat has been removed.
Cut out any jagged edges you may have missed with your Exacto knife. These small jags of mat are typically in the corners of the window you just cut out. Sand down the edges you cut with some sandpaper to smooth out the look of the mat.
Align the picture in the mat window. The picture's edges are going to slightly overlap the mat, giving you room to tape down the picture. Use masking tape along the entire border of the picture to firmly hold it into place against the mat.