Start with your A3 sheet of paper and a pair of scissors
Fold the bottom left hand corner of the page so that the short left edge of the paper lines up with the top long edge.
Fold the remaining rectangle of paper over to the left.
Cut along the rectangular fold line. This is the last fold you made on the paper.
You now have a perfectly square piece of paper. You can use this method with any rectangular piece of paper. The size and proportion of the rectangular piece will vary.