Here is the fine horseman Warwick Schiller getting ready to laterally flex his horse’s head starting on the ground. He’s going to slide his hand down the halter rein, and when the horse gives his nose, he will release. Pretty soon it will look like this, if you release as soon as the horse gives to your suggestion:
The horse should willingly bring his nose and face around in a vertical position. There are many videos on youtube showing all the steps. Here’s how it should look, in both directions, before you ever try it from the saddle. More on this coming soon.