Here is how:
Avoid performance pressure about the "magic spot".
People can become fixated on achieving a sexual goal (multiple orgasms, simultaneous orgasm, g-spot orgasms) One thing I can promise is that this is the best way to NOT enjoy any sort of sexual encounter. Remember that sexual exploration is mostly about the journey, not the destination (although the destination is better than most, I will agree). Try not to make this another notch in your “sexually self-actualized” belt.

Turn yourself on.
Any homework that starts with this is bound to be somewhat fruitful. The spongy area around the g-spot gets engorged with blood when you are sexually aroused, so it is much easier to find and feel when you are turned on.

Get comfortable and find the g-spot area.
Lie on your back, squat, or lie on your stomach. Place your palm face down on your vulva and slowly insert a finger inside your vagina (use lube if you are feeling a little dry), crooking it forward in a “come hither” motion. When you are up to about the second knuckle, you should feel a slightly bumpy or ridged area on the upper wall of your vagina.

Notice how the g-spot feels.
The texture of the g-spot area will likely be noticeably different from the typically smooth walls of the vagina. When you are aroused, it can expand, so feel it at different times during your arousal to get familiar with its contours and sensitivity. The g-spot responds to pressure, so press down and pull forward using that "come hither" motion with your fingers.

Explore the g-spot with toys.
For some it can be awkward to stimulate the g-spot by hand. A g-spot vibrator or dildo can be a great helper in this. Apply a little lube to your toy, and insert it with the tip (if it is curved) pointing up toward the top wall of your vagina. Work it in slowly, far enough (a couple of inches) so the tip is pressing against your g-spot.

Experiment with pressure and motion.
Some women will find pressure against the g-spot pleasurable, some women like the feeling vibration when a toy is pressing against the g-spot. Experiment and see if either feels good for you. For most women, the g-spot responds to firm pressure. In the beginning, use your toy as if you were trying to scratch an itch—don’t pull the toy all the way out, but use short strokes, applying firm pressure, against the g-spot.

Vary the movements.
A circular or back-and-forth motion may be necessary to get you started, but you might soon graduate to a more vigorous thrusting. If you have a vibrator, try playing with the vibrations both on and off to see which you like better.

Add clitoral stimulation to g-spot play.
You will know you are hitting the spot as you feel tingly sensations, the urge to pee, and an overall elevation in your arousal. When you feel the urge to come, stimulate your clitoris using your favorite method. Keep stroking your g-spot.

Let go.
With continued stimulation, you will eventually feel a sensation much like having to pee. This can be quite disconcerting at first, and has probably led plenty of women to abandon the process, but if you stick with it, you will be in for a pleasant surprise. You may or may not ejaculate, but ejaculation is perfectly normal (and it is not urine).

If you do not at first orgasm, try again. It can take several practice sessions before you notice any build up. Try varying your position, using a different toy, experimenting with breathing and kegel exercises (to strengthen your PC muscle), or having a partner help you. Because the g-spot is most responsive when aroused, you may also want to try stimulating it after you’ve had an orgasm.