I've only tried this with one type of Christmas lights so if you try it with other types and it doesn't work... Well then that sucks.
Anyway this is really simple and won't take long at all. Maybe like ten minutes tops if you include searching through drawers for stuff like batteries and cleaning up a broken lightbulb.
Step 1: Stuff
Step 2: What to do
Step 3: All done :D
Just a word of warning, once you cut off the plug do not connect the lights to it again and try plugging it in. My mom told me that