NazzNegg made some great points, Also helps to have a great tongue lol
Here's some pointers for you!
There are many ways to use your fingers to make a girl go crazy, and some of the best are to be found by simple exploration - try different things, different numbers of fingers, different parts of her pussy and ass, and be innovative and inventive. Some of the best fingering comes in conjunction with oral... but there is one, shining act which transcends all sexuality, and gives a woman - any woman - the best orgasm of her LIFE. If I am sharing this advice with you it's because I think you're a nice guy or girl. This is not the kind of knowledge which should be bestowed upon predators or misogynists. You should only know this and use this if you LOVE women.
Here goes:
Start by performing a little oral on her inner thighs, labia and clitty especially, to get her nice and wet. The rest of this (except for, perhaps, the very end) can be enhanced by continually sucking and licking the clitoris throughout the duration of the act - oral definitely increases the power and pleasure or the female ejaculating orgasm.
Now, take the hand that you write with, and take the two fingers closest to your thumb (your pointer/index finger and your middle finger) and make them wet with your mouth, with hers, or with her pussy juice (she should be glistening wet for this to work properly). MAKE SURE YOUR NAILS ARE TRIMMED AND NOT SHARP.
Slide those two fingers in, with the fingerprint part of the fingers facing up (nails down), slide them in firmly but slowly, as far as they will go without using too much pressure. If she is really tight you make need to start with one, and if there is a lot of wetness but resistance, you may need to tease and taste her pussy some more. Once you get those fingers in, tease her inner walls slowly with the pads of your fingers, and get her even more juicy. When you think that she is ready to have the orgasm of her life, you may begin:
Now you will maneuver your fingers to find her g-spot. Yes, it exists, and yes, every woman has one. Slide your fingers in about 2 inches, UP, and then BACK towards the front of the pussy (like you're going up behind her clitty). Her g-spot is actually a patch of her inner walls which is on the FRONT INNER WALL of her pussy, above the hole itself. So, in, up, and back towards the entrance. You should feel, on that wall, a very rough patch of skin