Get a 2 joited fulmer. This will prevent the bit sliding thru the mouth as she gets used what she is supposed to do when pressure is applied.
Before you ride her in the bit, drive her. This will ensure that she understands left, right and stop. You are lucky that she is backed already and can be ridden in a bosal.
Here is how I would approach the situation. Bridle her and let her loose in a round yard or similar, so long as there is nothing to catch the bridle on. Have the cheek pieces long when you put the bridle on and slip it over her ears, so its not uncomfortable for her. Then tighten them so there is just one crease in her mouth. Then let her loose like this for about an hr, morning and evening if possible. Repeat this for a few days, then gradually start tightening the cheek peices until you have 2 creases. Then you can also add side reins and a surcingale. Don't tighten them, just have them so that the horse can feel a slight contact while her head is in its natural position. Let the horse walk around and figure out what to do with its head, but don't leave them on too long and watch the horse the whole time.
I allowed my horses to chew soft lucerne during this stage (when not in sidereins), but some people don't agree with this method.
Once the horse is comfortable with having the bit in the mouth, add reins. Stand at the shoulder, and ask the horse to flex to both sides. Once she understands this, proceed to driving the horse. When you begin driving, the most important thing to teach is whoa. Once that is established you can work on turning and patterns at a walk, and circles at the trot. Once the horse is relaxed during this process and has a good understanding of what you are asking, try riding. Again, establish woah. Keep the lessons short and simple at first, don't over load the horses brain.
Its important that throughout the whole proccess you are soft on the mouth. Ask and release, ask and release, until the horse gives to the pressure. Then praise. Don't ever put the horse in a situation where the presure can't be released when the horse gives to it (eg tight side reins).