How can I divide full sheets of sandpaper into the sizes I use without folding, creasing, and back-creasing, then tearing on a straightedge?

Fashion a sandpaper ripper with a piece of plywood and an old hacksaw blade. Measure and mark the sizes of paper you normally use on the plywood surface. With wood screws, mount the blade and use thin washers below the blade so that the sandpaper can slide under it easily.
—From the WOOD magazine shop