Alright, I'm too cheap to spend $5 on a plastic extension cord wrapper to keep my Christmas lights from becoming a tangled mess over 11 months in storage. I had a heaping pile of shipping boxes from all my on-line gift purchases though, and I had to do something with them anyway. Maybe it's an obvious solution, but sometimes those are the ones that get right by us.
Step 1: Measure, Cut, and Fold
Cut a flap or side of a shipping box, from about 6 to 11 inches long and 4 to 6 inches high. Orientation of the cardboard corrugations is important here, we want to keep it vertical so you will see that wavy "S" pattern in the long measurement of your rectangle. This adds strength to the outside "ears" of the wrapper.
Make two inward diagonal cuts on each long side of the cardboard, about an inch in from each end (see diagram). These cuts will allow us to fold the cardboard into thirds and result in a flattened spool shape.
Step 2: Time to Wrap
Start with the plug end of the string of lights. Fold the plug cord in the fold of the cardboard and wrap the cord around the spool by grasping the ends and turning end-over-end (this prevents twisting the cords). Depending on the initial size of the cardboard and the length of the light strands, you should be able to get at least two strands on a spool. Plug the cord into its self to keep it from unraveling.
All done. Now box these guys up for next year.