Well, to put it shortly.
1. Restrain the guy somehow (well it's actually quite easy sometimes) and fixate him.
2. Take this clothes off, or just unzip it (whatever).
3. Since they're made of flesh and blood, and they have their instincts and stuff, there's a really good chance that the guy in question will have a hard-on even if you don't stimulate him.
4. Make yourself at home.
5. Move your body, fuck yeah.