The horse should lunge in the saddle and bridle (with stirrups hanging down) in a relaxed manner.
“The first rider on a horse should be elastic, athletic and fearless.”
Start with knee bends then little jumps up & down next to the horse, gradually making jumps higher and high on both left and right of horse.
While on ground, use hands to put pressure in the stirrup by pulling down on both sides.
Standing on left side next to stirrup, rider should put her right arm straight up above horse’s croup, more and more quickly. This prepares the horse for the rider swinging her leg over.
Next, put the arm up, hit the saddle lightly to make a nose, then put the arm up again. This is similar to someone posting at the trot.
Practice all these on the ground for several days until the horse accepts everything.