Like mentioned above you need to establish proper direct reining cues first where your horse follows your direct rein with its face and shoulders,
Then if you want to teach your horse to neck rein you teach them a desired response to a stimulus. Like what was explained in other comments, put the rein on their neck then use the direct rein to show them the response you want. You can do the same thing with any other type of stimulus as well, for example use a clicker and click twice and pull left pretty soon the horse will respond to 2 clicks by going left. The problem with this is that if the horse decides not to listen to the stimulus and they will at times you have no way to make them turn but to pull.
If you want to teach your horse to guide one handed that is very different you start by riding each shoulder with with one of your reins and then making your horse respond off lighter and lighter contact until you can move both reins with one hand and they respond to the contact. To do this you have to develop your horses self carriage and develop your hands and legs to work with the horse in rhythm. All my horses will guide one handed at a certain level and I am always trying to advance that level so if they decide not to work off such light contact I can just use a little more contact and they respond and don't leave me hanging out in the wind yelling turn dang it when I put the rein on their neck and they do n not feel like it. :o)
Get your horse between your hands and legs and then they will guide one handed, you can check out my coaching blog to see how you develop the foundation needed to guide one handed. Rod Miller IPHDA & NRHA Professional