Measure the dimensions of your picture with a ruler, and subtract 1/8 inch from each side to allow the mat to overlap all the edges. For example, if the picture is 8 inches by 10 inches, the inside dimensions of the mat are 7 3/4 inches by 9 3/4 inches.
From the dimensions of the inside of the mat, add at least 2 inches for the outside size of mat board. This will give you 2 inches between your picture and the frame. Adjust this dimension to a standard size for the outside of your mat to avoid also making a frame.
Draw the dimensions for the outside and inside of the mat onto the back side of your mat board. Use a sharp pencil and a ruler for straight lines. Check the corners for squareness and accuracy by simply holding a sheet of paper against the corner to compare.
Hold your mat knife or X-Acto knife blade firmly in your hand as you would a pencil, and place the edge of the blade on the pencil mark at the corner farthest from you. Place your ruler against the line, and drag the knife along the edge of the ruler toward you, stopping on the corner of the next side. Do this for all four cuts.