I use quite a number of paper squares for some of my origami projects – boxes, envelopes, Christmas decorations, seed packets, seedling pots, flocks of birds, bunches of flowers ..... This is how I cut them easily and accurately, and most importantly quickly. It is especially good for recycling magazine pages which often have a raggedy edge and would otherwise involve measuring and fiddling.
Start with any piece of paper with at least one right angle and two straight sides.
Fold the right angled corner in half, lining up the two sides that had formed the right angle, precisely. (In the example, fold the left hand side down to meet the bottom edge).
If a square with particular dimensions is desired, measure the desired side length from the folded point along the lined up straight edge. (In the example, measure along the bottom edge from the point at the left hand side)
Line up the matched edges with the bottom edge of a paper cutter, the grid lines of a cutting mat or one side of a t-square. If a measurement was marked, line the mark up with the cutting line. (In the example line the bottom edge of the paper against the bottom edge of the cutter) CUT!
A perfect square to play with!