Set a self-healing cutting mat or a piece of cardboard on your worktable to protect the surface from your knife blade.
Determine the exterior dimensions of your mat board by the size of your picture frame. A picture frame's listed dimensions are its interior dimensions, so an 8-by-10-inch picture frame will require an 8-by-10-inch piece of mat board.
Lay the mat board face-down on the protected worktable. Measure from one or two outside edges to obtain the desired exterior dimensions, and carefully connect the measured points using a pencil and ruler.
Unscrew the grip at the tip of your crafting knife and release the old blade. Wrap the old blade in masking tape before discarding. Insert a new blade, and tighten the grip around the blade.
Cut the mat along the exterior lines you have drawn. Hold the crafting knife as you would a pencil. Keep your wrist straight while cutting, so that your fingers, hand, and forearm are aligned. Move your entire arm when cutting, keeping the blade aligned against the edge of the ruler.
Cut the interior dimensions of the mat board to make a window for your picture, artwork, or document. To determine the side margins, subtract the width of the image to be framed from the width of the entire mat board, and divide by two. Repeat this procedure for the top and bottom margins, but add one half-inch to the bottom margin (subtracting a half-inch from the top margin). This will create visual weight at the bottom of the frame---otherwise, the framed image may look as though it is falling to the bottom of the frame.
Lay the trimmed mat board face down, and measure in from the four outside edges as determined above. Connect your measurement points with a ruler and pencil.
Cut the window, beginning at one corner. Hold the crafting knife vertically over the corner of the window, and pierce the mat from above with the tip of the knife in the corner. After the knife has entered the mat board, rock the knife back to an ordinary cutting position, and continue cutting as described above.
Center the image in the cut-out window, and secure the document or artwork in place with acid-free self-adhesive mounting corners. Place the matted image in the frame.