Don't start with this

With her on her back, slide two fingers inside her vagina, palm up. Not very deep the G-spot is only a few inches inside along the front wall of the vagina. You should feel an area that is slightly enlarged, with a spongy texture. The G-spot is not a smooth as the surrounding vaginal walls.

When you have located the G-spot, SLOWLY stroke the area. Pull your fingers back towards your palm curling them like you are motioning to come here. This area is extremely sensitive for most women, make sure you start softly and slowly. You can gradually add a little more speed and pressure according to her responses. After a little stimulation the G-spot will begin to swell and enlarge.

Clitoral stimulation will assist in her reaching orgasm and ejaculation. Use your mouth or other hand for this purpose.

Vary your stroking of the G-spot according to her reactions. The fingers can rub the area together or you can use them separately as in a walking motion. Another technique is to pull the fingers out in a motion similar to intercourse. Lock your fingers straight and at an upward angle toward the G-spot. Use very short strokes about an inch back and forth over the G-spot. Don’t remove your fingers from the vagina.

Be very observant of her reactions. When you feel her vagina contract and she starts to bear down pick up the pace. Start working your fingers more rapidly. Placing your free hand on her mons, the meaty area above her clitoris where the hair grows may also help. The slight pressure helps isolate the G-spot. Encourage her to relax and let it come.

As she gets closer to coming, she will start to feel as though she has to urinate. That’s the cum beginning to flow into her urethra. In order for her ejaculate, she has to relax, let go, and push down and out with the same muscles she would use as if she were peeing. It may be hard to overcome the impulse to hold back, but you can reassure her, again, that if she is coming, she will not pee. 
Instead, as she comes and pushes the fluid out, she should feel extra intense orgasmic pleasure, and you should be rewarded with a spurt of nectar that may range from gentle gush to a drenching spray. 

If you are unsuccessful with this position you can also try doggystyle. Position her so that her head is on the bed or lower than her hips for a better angle at the G-spot. Use the same technique as used in the previous position. Pressing down on the G-spot instead of up.

Enjoy each other and have fun.