You just start neck-reining the horse. Cue with outside rein and outside leg pressure, reinforce with old plow-reining after a short delay. Combine the new cue with the old stuff. They pick it up pretty quick.
I mostly followed this video. It was posted on a different thread months ago and it makes sense. My mare already responded to very light leg cues and figured it out extremely quickly. And this guy is just fun to listen to. Haha.
I dunno...I kinda just did. They knew how to respond to leg and seat already, and I rarely kept contact with the reins anyhow, so I just started neck reining. I don't think I did it properly, 'cause sometimes they need a little bump on the mouth, but I can ride my one mare with just a neck rope, so I guess I did something right.