Mount. Stand square. Do flexions (i.e. sideways bends) of the neck to distribute the weight evenly. The low neck can do full flexions. I.e. the left ear can come around as far as the rider's left knee (but should not come around further). The raised neck can do half-flexions. The higher the head, the smaller the range of flexion. The head, nose pointed towards the stars, can not set the poll sideways. In all flexions the ears must remain at the same height.
Walk forward energetically, reins long. Take up the reins and ask the horse to gradually raise the head. All the way until the nose comes into the horizontal (a note on physiology: i.e. both, yes and no joint close and the long ligament is released completely). The higher the head the shorter the steps, until the horse picks up the so called counted walk (i.e. a very slow, yet high energy walk).
Give the signal, but neither support nor force the horse. Give the horse the opportunity. To avoid restistances raise the quality of the demand. Work towards the horse's conscious and willing release of the jaw. If necessary use the taps of a small whip on the shoulder to demand the horse's attention and input.