First, indoors, mist the window with water. Next, cover the entire pane with a piece of wrap, bubbles toward the glass. (Cover only the window, not the frame.) Press and hold in place briefly so the wrap adheres. Don’t pop the bubbles! It’s the air inside that has the insulating effect.
Extra tip A: use wrap with large bubbles. They’ll let in the most light.
Extra tip B: add a few drops of bleach to your water before spraying the windows. This helps prevent mould.
Bubble wrap is sure as heck no substitute for new double-pane windows…but it is quick, easy, and cheap.