A quick study of teaching vertical flexing from the saddle. Begin by adding lightness of pressure with the reins pulling toward the outside your lower thigh more toward your knee so you aren’t pulling up on the reins (if the horse moves you are adding pressure to quickly) and instantly release when the horse tucks its nose. The release has to be instant.
That being said, this is only one step. There are many more you need to have in your bag. Remember, teaching horses is a process that has to be taken very slow. This will give a place to start in the saddle.
I would recommend actually beginning teaching the give to pressure from the ground as part of your ground play or concepts.