Heather Corinna replies:
How's sex done?
Any way you do it, however you define it.
In other words, what "sex" even IS varies pretty widely from person-to-person and day-to-day, and can be or include ANY number of sexual activities. Intercourse is sex, but so is oral sex, anal sex, manual sex, making out, frottage, role play, cybersex, mutual masturbation, masturbation...and just about anything else you can dream up in which one or more than one person is doing something seeking or experiencing sexual gratification.
And how any one person or more does those given activities also varies: by position, by how people are touching each other, what body parts come into play, the works. Sex between people is basically improvisational dance: how two people do something is (and should be) about what feels good to both of them, with their unique bodies and preferences, and how both uniquely work together.
So, in short, you asked a much bigger question than you perhaps meant to ask. Your best bet? Peek around the whole site here if you're thinking about one particular sexual activity. Experiment with and talk with your partner -- after all, as a people, we've figured out the basic mechanics of these activities before we could even read, the mechanics really are pretty intuitive and uncomplicated. Explore, and ENJOY the exploring: that's one of the best parts!