Place your mat board on a soft, stable surface. The best cutting surface is a large drafting table that is covered in a simple carpet to keep the mats from slipping and protect the blade as it penetrates the mat board.
Trace an oval shape onto the front of your mat board using a light pencil. Ensure that the oval is large enough to show the picture properly but not so large that the edges of the pictures are revealed.
Cut the oval shape using a hand-held mat cutter; Dexter and Logan are two of the graphic-supply makers that offer the device. Be sure the bevel of the blade is in the right direction, if you are cutting your mats at an angle. Usually, the bevel is at a 45-degree angle to the base of the mat board. This technique can be tricky, so be sure you practice cutting with the device on scrap mat board of the same thickness prior to beginning your final project.
Move the device slowly around the traced oval. If you find that staying on the traced line is a challenge, you may want to find an oval shape made of wood or plastic and trace around the outside of this object to keep the blade from straying off of the traced line.
Check your cut to see that the edges are smooth with no fraying of the mat board. If you see some rough edges, smooth them with an eraser or a soft surface. If the fraying is dramatic, use a sharp X-Acto knife to cut the excess mat board.