Illustration board is a good, strong backing to use to mount your artwork. However, because it is so strong, it can be difficult to cut down to your preferred size. It can be tricky, but if you have a steady hand, your illustration board will become the perfect frame for your masterpiece.
Place your cutting mat and piece of illustration board on a cleared and sturdy table, preferably one that is about waist-high to you.
Lay the reverse side of your T-square along the place you are planning to cut. The head should be resting over the edge of your illustration board and the blade (the long part) should be laying completely flat.
Draw a line on your illustration board along the blade of the T-square to mark your cut.
Cut along the line (and the blade) with your box cutter. Push down on the T-square to give yourself more leverage.
Turn your illustration board over and place the T-square along the crease from the cut you made on the reverse side. Complete your cut by running your knife along that crease.
File down the cut area with the piece of sandpaper to smoothen out the edges.
A yard stick or a straight-edge ruler is a viable substitute for a T-square. The ideal place to cut illustration board would be on a drafting table. Remember to use a few light, slow cuts when cutting your board. That way, the edges are cleaner and you don't have to spend as much time sanding.