Here are the four basic G-Spot stimulation strokes as outlined in Female Ejaculation: Unleash the Ultimate G-Spot Orgasm. Just remember that any G-Spot stimulation works best if the urethral sponge is engorged, which happens with sufficient arousal.
IN-AND-OUT: This is pretty basic stuff, basically you want to slowly insert a finger into your well lubricated and aroused vagina. Slowly work up to more than one finger or even your whole fist if you are so inclined. As your vagina opens itself up with further arousal you simply want to explore the depth of it with an in-and-out motion.
HOLDING: Holding can begin with a cupping of the vulva with the palm of your hand, and when combined with the in-and-out stroke refers to finding the engorged area of your urethral sponge and holding your finger (or fingers) on that spot. Depending on your level of relaxation, you might want to hold your G-Spot for ten to thirty seconds and wait for signs of relaxation and comfort before proceeding further.