I always say the trick to neck reining is - not to neck rein, lol. A lot of times people hop up there and start pulling that rein against the neck and the horse bends in the opposite direction of where you want to go because that's what you're telling him.
"Neck reining" in western pleasure terms, is a horse that is so off the bit you pretty much only use your seat and your legs to control the body of the horse.
With that being said, start out direct reining in a snaffle. From the assistance of the snaffle, begin to learn to move the 4 "sections" of your horse's body. Section 1 is the head/neck. Should be able to flex to his shoulder with a pinkie's worth of pressure. Section 2 is the shoulder. The shoulder should be able to lift and cross over at your cue. Section 3 is the barrel. You should be able to lift the barrel, or bend it left/right. And Section 4 is the hindquarters. You should be able to manipulate and cross the hindquarters.
Putting these things together come from a series of flexing, bending, counter bending, circles, counter-bending circles, two-tracking/half-passing, side-passing, pivoting, turning on the forehand. You should start from the standstill, then to the walk, and work up to the jog and lope at these exercises.
Being able to manipulate the core will help your horse engage through the straight, slow, and drop his head naturally.
Once your horse listens to your seat and leg with each body part, and you've begun to drop contact on the snaffle to do these exercises on a draped rein, you would be ready to move on to a medium port curb.
My biggest advice would be to first read AQHA rulebook. Just to familiarize yourself with the sport and its rules. Then I would suggest looking into maybe getting some lessons, going to a clinic, watching WP demonstrations, etc.