The back kick (sometimes referred to as a "mule kick") is a strong and easily executable attack in the martial arts.
Stand with your feet slightly further than shoulder-width apart. Do not lock your knees; this is unwise at any time during a fight (unless you like expensive hospital vacations).
Now you want to lock-in on your target. Turn your head over the shoulder above the leg you will be using. So if you're going to kick with your right leg, look over your right shoulder. Do this without moving your feet.
You are properly positioned and your eyes are on the target, so it's action time! First you want to chamber your kicking leg. Do this by pulling up the leg near your chest. Get your knee as high as is comfortable. (This part will come more naturally as you have executed more of these types of kicks.) As you are chambering your kicking leg, lean your torso forward a bit. Essentially the further you lean forward, the higher your kick will go.
Tip: Visualize this part as compressing a strong spring.
Now just shoot your leg straight backwards into the target!
Notes: Contact is to be made primarily with the heel of the foot. Being a strong kick, you may want to kihap while doing it. Also be careful not to "lock-out" the knee. That is, stop the kick with your muscles so that the joint doesn't have the burden of all that pressure. Also keep in mind that this kick, like every other kick, is only effective with practice. Being simple, however, this kick should take little work to make it into one fluid motion of destruction.
There are numerous applications for this type of kick. One of the more obvious and practical applications would occur if you were in a fight against multiple enemies. When Guido thinks he's sneaking up on you, send your heel into an open target on his body and show him what you think about being outnumbered. If anything else, it would at least be a defensive measure to keep Guido from getting too close to your back.
Although risky, this could also be used as an offensive attack while feigning flight. You could turn as if to flee, possibly even run a step or two, and then put a massive amount of energy into your pursuer's groin, gut, knee, or whatever else is in range.