i cut up some icicle lights last year for my arches. sometimes the cut was down a icicle.
1 the best way to find the end is to pull out the first bulb.
2 plug in the light set
3 look at the first bulb that's lit (this is your second section)
4 I put a piece of electrical tape between the last darkened bulb, and the first lit bulb.
5 unplug the light string
6 put the bulb back in
7 unplug the first light after the tape mark
8 plug the light string in and see if there are more sections
repeat 4-8 as needed.
when finished MAKE SURE string in unplugged from wall, then make your cuts at the tape marks
(be sure to leave enough for your splices)