This is a simple but useful technique for folding a square out of a rectangular piece of paper. It's vital for anyone who wants to create origami out of photocopier paper. I've included simple ASCII drawings to make it easier to follow - please forgive my lack of artistic skill.

  1. Line up the rectangular sheet so that the long side is horizontal:
    |      |
  2. Fold either of the upper two corners down, so that one of the short sides is lined up perfectly against a long side:
    | | /
  3. Finally, fold the bit of paper protruding from under the folded triangle up. Now you have a square!