Trim the board to size along the lines drawn using a utility knife. Make sure that you put your straight edge on the side of the board you wish to keep...that way if you slip it will be into the waste.
Remember that the utility knife cuts along its flat surface not the tip so you should have a shallow angle between the knife and the board. That way you are presenting the maximum cutting surface onto the board.
Note that it is easiest to cut the board in several passes. Scoring first and then gradually cutting through with each successive cut.
You can now draw up your window size on the back of the mat by measuring in from the edge of the board. |