Most people lift their legs when they fart, however, this reduces the noise normally because they also move there leg away. To achieve the greatest volume, stand on one foot and bring the opposite knee toward your chest, also slightly outward, creating a seal. Wait until you feel the pressure will be the greatest, then forcefully release. Do this in public, I assure you people around you will be sure there is an elephant or horse nearby.
Other tips: Do this in areas which have a large amount of elderly people: it is well known that among the elderly that old people have no control.. yet farting remains socially unacceptable, ESPECIALLY if the farter does not confess. No one will suspect the only teen.
Also, I recommend to increase the foul smell, eat LOTS of cauliflower the day before, this will generate much smell. I recommend AGAINST beans, because although beans produce more gas, they do not smell, so what is the point. Cauliflower produces farts that reek. Do this and fart away in crowded restaurants, no one will know it is you and even if they find out, WHO CARES since everybody farts even if they don't admit it.