Teaching a horse to “neck rein” will depend on what you mean by “neck reining”.
Professionally trained horses end up neck reining because they have been trained in a way that eventually they are riding one handed.
If that is the kind of neck reining you mean, you train the horse well in all you do with all aids and use hands less and less, horse is in self carriage more and more.
The horse learns to respond to all other aids to the point that you don’t hardly use your hands.
You end up where you can barely move your hands an inch or two and the horse moves, then go to one handed riding and there you have one kind of neck reining.
If you want a horse with less technical training to respond to being handled with one hand, no matter how the horse is working or what else is going on, as in speed events, then that requires other kinds of neck reining.