I use a rubber sanding block and palm sander for final sanding of project parts. But cutting full sheets of sandpaper to the proper size for each one was little more than guesswork.
To make this a foolproof task, I built the cutting jig you see here. It consists of a base and a three-sided fence. The cutting guide is sized to fit against the side fences. The guide is reversible for cutting either 1⁄3- or ½-size sheets. The detail drawings below show how it works.
To use it, slip the paper against the fences, fit the guide against the fence, then cut along the edge.