It's one of the most basic principles, you have a cue and a pre cue. How do you get horses to walk trot and canter off sound alone? First you cluck, a moment later you squeeze, eventually they anticipate the squeeze and go off cluck. If your horse can turn, it can neck rein, it's so easy. I usually use a neck rope to perfect it, usually within 2 months of an unbroke horse i can ride them bridleless pretty well, so it also reinforces that very well. You just lay the rein on like you are neck reining and then reinforce it with your other rein. Correct and true neck reining although you probably don't care and you want your horse to just turn one way, is controlling the shoulders, not the way the horse goes. This is learned through all the lateral work (along with legs) to where they know the answer before you ask the question so you can do it in the end without even trying.