The G-spot, on the front side of the vagina in the first two inches is what gives vaginal orgasms. It needs to be excited ("warmed up") before it can contribute to an orgasm. Like a penis it gets engorged with blood and becomes more sensitive when aroused. Inserting one or two fingers into the vagina and moving it/them against this area helps cause this arousal.
I've found two fingers, angled a bit forward in her, with one in front of the other rather than side-by-side is good, because flicking them against each other, so the one in front rubs against/stimulates the G-spot, works well.
You can feel when the G-spot gets engorged, it gets sort of rough and swells some, filling that part of the entrance to the vagina more. She should feel it too -- as arousal.
It won't be instantaneous, but it may happen. While doing the G-spot stim, you can use your thumb on her clitoris, and even your ring finger knuckle against her anus (yes, that can excite her too). Those three spots can help her get excited -- sort of like you getting an erection -- which makes vaginal orgasms much more likely.