- 1). Turn the picture frame over and measure the dimensions of the glass. Transfer the measurements to the back side of the mat board. Draw a cut line for height, and a cut line for length using a straight edge. The cut mat board must be the same size as the picture frame glass. This means you need to cut the mat board to size out of the larger sheet of mat board.
- 2). Position the mat board face down over a self-healing cutting board. Place a metal straight edge along one of the cut lines. You should see a line away from one edge of the mat board representing the height of the picture frame glass, and a perpendicular line away from an adjacent edge of the mat board representing the length of the picture frame glass. Hold the straight edge down securely over one of these cut lines.
- 3). Drag a sharp artist's knife along the metal edge. Don't press. Allow the sharpness of the blade to cut through the top layer of paper. If you press, the straight edge will tend to move, causing an incorrect cut. Cut over the same cut several times until the knife cuts through to the cutting board. Repeat this for both cuts. This will produce a mat board the same size as the picture frame glass.
- 4). Tape the mat board down on a clean table face down. Line a t-square up with the closest edge of the table. Position the mat board against the top edge of the t-square. Tape each corner.
- 5). Measure your picture or image. An octagon is a square with clipped corners. Determine the size of the square. Measure, center and draw the square onto the mat board in pencil. Use a 45-degree triangle with its horizontal leg pressed against the t-square to draw the vertical lines.
- 6). Divide the width of the square into thirds. Divide the height into thirds. Draw horizontal and vertical lines inside your original square. This will create three rows across and three rows down for nine total squares. Start at the top left. Move your pencil to the right one square and mark. Move your pencil from the top left and down one square and mark. Use the triangle to connect the marks. Repeat for each corner moving away from the corner and up or down from the corner. This will create an octagon when you are finished.
- 7). Place the mat board under the straight edge of a mat cutter. Line up a pencil line of the octagon with the straight edge. Press the mat-cutting tool straight down into the mat. Push the tool forward while pressing down on the straight edge. Stop at the intersection of the next octagon line. Turn the mat. Line up the line and the straight edge and cut. Continue all around the mat.
- 8). Remove the mat, being careful not to dislodge the inside piece of mat board if it is still attached. Turn the mat board over. Often the knife will not cut all the way to the corner because the blade is angled. Use the artist knife to follow the cut to the corner. Slide the knife into the mat board at the angle of the original cut and gently slice toward the corner. You may have to cut both sides of the corner. If you don't cut the paper, it is likely to tear, creating a blemish.
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