Do I just cut the and hook it to the battery? It can't be that simple.
It was that simple well over a decade ago when we were living without power.
Can you offer more detail? Holiday lights have a 120-volt plug?
If so, a small inverter would give you 120 volts from a 12-volt battery.
An inverter would be the easy but inefficient option. Cutting the wiring and hooking to the battery could work depending on the power range of the LEDs.