Robin, first have your vet check your horse’s teeth—she might need to have her teeth floated. Assuming her teeth check out okay, this sounds like a great opportunity for positive reinforcement training. Begin by teaching your horse that a particular sound or word means that food is about to come. I like to use a short whistle sound that I can make with my mouth. Make the sound—let’s call it a “tweet”—and then give her a small, tasty treat. Once she is anticipating the treat when she hears the sound, she is ready to learn!
I suggest training her to first touch a target (a plastic soda bottle will do) with her nose for a food reward. Place the soda bottle about two inches from her nose, and when she reaches forward to touch it—tweet and treat. Repeat until she reaches forward a couple of feet to touch the bottle. Slowly increase the time she has to keep her nose on the bottle before you tweet and treat. Now you will teach her to put her head through her headstall to touch the bottle—essentially you will be teaching her to put on her own bridle!
You may need an extra set of hands when you begin. Place the headstall between the bottle and your horse. Have her reach to touch the bottle—having her touch the bit on her way. Now, require her to open her mouth to accept the bit before she touches the bottle. With a bit of patience you will be able to teach her to keep her head low and steady and open her mouth when presented with the bit.
Have fun!