How do I retwist Christmas lights?
Use a spool to store them
They make a wide variety of wire caddies and spools intended for spooling up extension cords.  My &quot;go to&quot; is about 18&quot; long, flat, and looks like this:
   ------+--------------------------+------
         |                          |
   ------+--------------------------+------
                  \------/  

That thing being a handle.  You attach one end (they give a little hook for that and spool up the wires.  That's great for storage and easy unfurling.
However, to re-twist, the spool is especially essential. As you're spooling up the wire, when you hit an untwisted area, stop short of it.  Rotate the whole spool.  That lets you add twist to the wires as needed.