See, we won't really be able to help. All we can do is give you the same general information about anatomy as Google. The problem with that is what you're flicking past is her cervix, and we don't know anything about how it is positioned. She may not know either, though she can certainly find out. Cervices are not all the same size, and vaginae are not all the same length. Her cervix may be further in, or more shallowly located than mine. It may point forward, back, straight down. It may occupy more space in her vagina than mine. She could move to a new phase in her menstrual cycle and THAT could reposition her cervix.
Now, for the same stuff Google will tell you. Warning: I'm going to write in almost purely clinical terms. You may or may not require a dictionary as some familiar words do not carry the same meaning in a clinical context.
This spot is the posterior fornix. It is located on the superior cervix, which itself is located on the anterior vagina. To reach, she must be very aroused so that the cervix will move out of the way. At this point, the superior vagina will change shape and size like an expanding balloon. With more length, more pressure could be applied. This is key, because that part of the vagina is generally far more sensitive to pressure than to temperature or friction.
For me, long and upwardly curved reaches most easily in missionary. For me, a partner must initially aim toward my backside, brush past my cervix, and then arch slightly forward. What I like best is to feel the glans pop back and forth gently past my cervix and then apply extremely firm pressure to the posterior fornix. Shay enjoy directing you herself by climbing atop and doing to herself what feels best, an act from which you can learn plenty for when it becomes your turn to ride.