I have trained all of my cutting horses to neck rein using this method:
First, place the reins over the horses neck and tie a knot, if there's not one already, to hold them together at the right length for you and your horse. Then take them back over the horses neck and criss cross them. Instead of putting them over the head so that they are straight and on the correct side of the bit, criss cross them then put them over the head. Walk the horse around a bit and turn him getting him used to the feel. Then when you mount, hold you reins in one hand. When you lay the reins against his neck to the right, it will pull on the right side of the bit, causing him to turn right. Vice versa for the left.