Very simple! :) Just takes patience and repetition.
Hop on, and go for a walk.
Say you're walking around with your right hand on the inside, and you want to turn left. Lay the right rein across the neck, then follow up with a touch on the left rein. Once he responds to a turn, release and walk straight. Repeat again, also making sure that your right leg is asking him to turn as well. Do this both sides.
A horse that neck reins really well should also have learned to move only his shoulders on command.
If you can do turn on the hind quarters, it will make your training alot more simple for the horse.
To get the best, clearest results only move your hand across perpendicular to the horse's neck, and with no angle like towards your hip. The higher up the neck, the more response you will get.
You want to make sure that your horse isn't leaning forwards when you turn. If so, back him up a step to re-direct his balance. If a horse is down in front, and you ask for a neck rein, he will most likely turn off his front feet, which gets more turn from behind than in front. So make sure his balance is on his hind end.
Once he understands that the rein across the neck means turn, you can start to fade away the use of the direct rein.
To help the shoulders move, you can carry a whip if your horse tolerates it, and at the same time you are asking for a neck rein, you can tap his shoulder to help him further his understanding on where he needs to move from.
Your horse should not have his head bent to the right, if you are trying to neck rein to the left. Maintain a straight line from his withers to his ears, and make sure he is bending his neck in the direction he is turning. If you ask for the neck rein, and he does bend his neck the opposite way, you just touch the direct rein to straighten him out. The more in-line he stays the better it will get.
Good luck, hope that explains enough!
In riding, a horse's energy is like a river- guided by the banks but not stopped by them.