So you're bored at the party, or you showed up more than fashionably late and decided to catch up quickly. Even better, the idea was suggested by a group and you've decided to partake. Shotgunning a beer is one of the many ways to add a great deal of booze into your stomach in a short amount of time. As usual with any such method, precautions should be made. I don't suggest doing this on a full stomach, or towards the end of a night of drinking, unless you like to make offerings to the porcelain goddess. Also,I highly recommend that one does this with cheap beer. Light beer will be easier to get down, but I find ice beer much more enjoyable. Do not use any good beer, for it will be a waste, and for the love of god do not use anything really thick and heavy.
First hold the can in the palm of your hand, so that the top (you know, the part with the tab) is angle towards your hip. I usually rest my elbow against my hip and hold the can as described.
Next grab a knife, one that's not rediculously large, but it should be sharp enough to easily cut through the aluminum of the can. Be careful with pointy objects when intoxicated, bad things can happen! If no knives are available, or if you prefer not to use something so sharp you can use your car keys. Take the keys or knife and cut a small hole at the top. Your mouth is going to go over this hole so be sure to cut it the right size. The reason the can is angled is so that when punctured, the carbonated contents do not spray you, your friends, your cat, or anything else in the vicinity.
So now you got a can with a hole in it, and hopefully no mess. Now the fun begins. Put down the pointy objects or keys and turn the can sideways, with the hole facing up towards the ceiling. Depending on your preference, place the tip of either index finger underneath the tab of the can, just enough so you have a good grip on the tab.
Now the preparations are all set. Place your mouth over the hole in the can and wait for the countdown. Once the count has been made, you'll want to turn your head and upper body to the side while popping open the top of the can, and chug! The beer easily flows out of the can and into your mouth due to the hole the tab made in the top of the can. This allows for a quicker imbibement than a straight chug and works rather similarly to a beer bong.
If all goes right, there should be little mess on the floor. I highly recommend that shotguns take place near a trashcan or a toilet, just in case someone can't handle the rush of beer. The trashcan is also a good idea for quick disposal of the used cans. Also, due to the carbonation created by the beer running into your stomach, loud belches will usually eminate after a round of shotguns, so don't be shy.
To add to what's below, the hole I make at the base of the can is a large rectangular one, a little bit less wide than my mouth. I find the larger hole better for two reasons, I'm a bigger guy, and I usually race with my brothers to see who can get it done faster. This larger hole helps.