The easiest way to do this is to find someone lying down, preferably asleep. Unless they have a particular fetish for it, most people aren't going to let this happen. You simply straddle their face and let loose a nice gas explosion. Pretty much what the title says. Obviously if they're unconscious when this happens you might just bring them out of it or give them nasty nightmares.
On the topic of fetishes, apparently it's a thing to have girls straddle your face and fart on it. I stumbled on this term, searched for the words, and honestly got a bit of a shock from the results.
Weird video of a dude's wife farting in his face accompanied by somewhat NSFW ads for finding other chicks to do the same to you in your area.