OK - don't start off with the g-spot stimulation.
Here we go - give her like 1 or 2 clitoral (ONLY) orgasms, maybe slightly introducing fingers in her on the second, then do the "come hither" technique as guided above with 2 fingers - the middle and one closest to little finger work best strength-wise (IME).
Keep some lube at hand, as her natural juices can run out quite quickly with the friction, once you get faster and faster working those fingers on her upper wall.
It can help to unleash teh power of squirting with a G-spot vibrator first, then once she knows she's not pissing herself, it's easier when you get the technique quite right.
Your technique might take a while to build up - A fair amount of getting it right is about what position she is in(I've found on all fours worked quite well when we first got my ex to squirt the first time, like a waterfall, using this
http://www.lovehoney.co.uk/bigpicture.cfm?p=2008&im=5602, but on her back N tilted forwards a bit generally works nice), and how relaxed she is mentally and physically.
I'll write more later, when I've thought more about it - it's been a while...
Also remember to pull your fingers out when she's cumming or it will come out in lil gushes and squirts, instead of a gushing waterfall/water cannon.