When N Kor attacks, the first sign will be the sound of motorized vehicles starting up way north of the DMZ. They will roll south joining up with other units as they go and cross the border in as large a mass as possible. If the northern movements aren't noticed right away, the attack will seem to begin abruptly and massively. As the troops approach the DMZ, massive shelling will commence, and that will include rockets. All offensive weapons will be used simultaneously. If there are any nukes available, they will be used in the first onslaught.
The defensive line in the south will try to delay the advance and fall back to prepared defensive positions when necessary. The entire operation will be over in three weeks, by which time N Kor will either own the entire penninsula or will have lost the war.
It all depends on how quickly the NK's can move south. If we can't move reinforcements over in time then things could get very nasty indeed. NK has been planning and refining it's options for decades and it's whole military revolves around the single purpose of a lightning invasion of the South.
I think it would be inevitable that a NK will have the upper hand in the early days of a war, but how quickly will SK/US turn the tide, that's the question.