The scissors position: She lies on her back, he lies on his side on her right (because most of you will be right-handed and he will need one of his hands to stimulate her clitoris, so use your better hand; if he is equally adept with both hands, then it doesn´t really matter unless his penis is not straight like a candle and hits her g-spot better on one side than on the other). She then pulls up her right knee to her chest, he also pulls up his right knee horizontally so that his right thigh comes to rest on her left thigh; his left leg passes under her left leg so that he can straddle it with his legs (the last bit is not a must, but this ensures that his penis wont slip out of her vagina during sex which can be annoying, especially when she is just having her orgasm).
This is it as far as the basics go. If left at that, however, it would just be a new position at best, but what´s so special at the scissors position is that it allows the male lover to give his female lover additional pleasure in many other ways while having sex, and this is what he should do if he wants to truly satisfy her.
First of all he has access to her clitoris - if it´s not as prominent or if the cock penetrating her is somewhat in the way, he can place the thumb of his right hand (if he is lying on her right) on the spot where her outer labia end and pull up, that way her clitoris will be drawn up and he will have easier access to it. He then can stimulate it by making small circles on it with his index or middle finger; only apply a little pressure and increase the speed the closer she gets to orgasm (of course it would be good to stimulate her clitoris before entering her, that way she will be wetter and find it more enjoyable; the same counts of course for foreplay).