Cutting a hotdog before grilling isn't required but it seems to cook or grill the hot dog more evenly when you do cut the hotdog before grilling.
You can spiral cut the hotdog either by hand or go online and buy a spiral hotdog cutter that cuts the hotdog into a spiral much faster and much easier.
I used to cut the hotdogs into a spiral by hand using a knife but after buying the spiral hotdog cutter it makes easier work of it and I make more of the spiral hotdogs.
I paid around $8.00 for my spiral hotdog cutter through Amazon.
I've had it a few years now and it's made easy work at making the spiral cut hotdogs.
But you can place hotdogs directly onto the grill which is what I used to do for years and they turn out fine that way as well.