Other tips:
make sure she is hydrated. This can seriously effect the amount of liquid expelled. I notice this when I am dehydrated its a trickle.
Empty her bladder before play so it doesn't feel like she will pee as well. Although that sensation is kind of normal with g-spot play
Some women like vibrations but a lot of us need short sharp fast hard jiggling thrusts against the g-spot. try both and see what works
Try it in the bath or shower first so that she can allow herself to "let go"
Expect a little pee to be released occasionally esp if she starts getting a full bladder. It happens, and as she learns the sensations, you can start to differentiate between the slight difference between "Oh I just squirted" and "oops I think I peed a little" Put a towel down and do not worry one little bit about it. The pleasure is worth it!
For me, when I reach "that point" when I know the intensity is at it's peak and I am there, i get a slight urge to bear down. In a similar way to peeing but also it is not the same. It is like I want to push out the toy or fingers inside my vagina. When I allow this sensation to happen and allow myself to push at the toy or fingers, that is when I ejaculate the liquid. This sensation starts to just take over, oh this is so hard to explain but what I am trying to say is to tell her to listen to what her body is saying and allow the feelings to come without trying to stop them or hold them back.
Try curved toys. I mainly use dildos. Vibrations are nice but won't make me squirt on their own. I jiggle the toy against the spot and curved toys just work so much better for me. I advise trying a curved toy. Nothing too fat or thick. Slimline is fine because this is not about filling her but hitting a spot inside her. Although, again, some ladies prefer thick. It is about finding your preference. Thinner toys may allow her to squirt even with the toy still inside as it doesnt "block" it so much (Like with fingers)
You may have to be "rougher" than you think. If you have watched it in porn, you will probably get an idea of the kind of short sharp fast stimulation that works for most ladies but again, find her preferred method.
When she gets to that "Oooh i feel like I want to push the toy out" feeling. Remove the toy quickly. In my experience I can still have g-spor orgasm or the same sensations over and over with the toy always inside me but there is not usually much, or any liquid expelled until the toy is outside of my body. I put this down to the toy "blocking" the urethra or something, stopping the "squirting" effect, although it can trickle out this way almost un-noticed until you check the bed after! lol
So whats it all about? Giving her some fast jiggling, shallow thrusting, speedy action and then timing your removal of the toy from her vagina at the exact moment that she is peaking and feeling the pressure built up to that point where she is tensing and bearing down. As she begins to sense that moment she can start to tell you. Otherwise, she can still experience those sensations only without the liquid.
Please let us know how it goes!