Lay the mat board face down on a sturdy surface. Center the print face down on top of the mat, leaving equal space around it. Using a sharp pencil, trace around the edges of the print.
Remove the print and set it to one side. Accurately measure the border, making sure that the top, bottom and sides are equal. Measure 1/8 inch into the top pencil line and mark it. Measure 1/8 inch from the end of the top line and mark it also. Repeat this with each line, rotating the mat when needed.
Place the metal ruler across the mat, joining the 1/8 inch marks. Draw a line straight across. Repeat for all four sides, forming a slightly smaller rectangle within the first. Using the metal ruler as a guide, score the inner lines with a sharp craft knife. You will need to go over the lines a few times, cutting through the layers of mounting board. Cut each of the four sides.
Gently push the cut rectangle through to the other side. If it does not release easily, carefully score the corners a little more with the craft knife. Lay the mat face down and place the print on top. Stick the print into place with masking tape on each of the four sides.
Place the mounted print into a picture frame if you choose. After the back of the frame is secured, the print is ready to hang.