Like others, I teach neck reining in a snaffle only. They move to a shanked bit once they have a full grasp of neck reining and any other advanced maneuvers required for that horse's particular discipline. Muppetgirl did a good job explaining how, I do pretty much the same. One step at a time, simply asking for them to move that shoulder over while in motion. The forward motion is a crucial piece. Like a car, you can't just turn the wheel and get where you want to go without any gas.
With a greenie that has no concept of neck rein but does direct rein & respond to leg I try to make it one smooth motion so they can connect the dots. Outside leg asking to move off of the pressure, light neck rein and open the door on the direct side. If they need a bit more encouragement a very slight lift of the direct to "show" them what you are asking with something they already understand. The neck rein is the "ask", the leg & seat is "tell" and direct if needed is "show/remind". With consistent leg, seat & hands they will put it all together fairly quickly and will eventually work well with any of the cues together or as individual pieces.