A side kick is also a standard self defense technique in many martial arts. Some martial arts refer to this as the back kick, which is actually a more accurate description. As my knowledge pertains only to Songham Taekwondo, my description shall be along the lines of that particular style.
First, start off in a middle stance (the kick can be executed from many positions, but for the sake of simplicity, we'll start here). Place your hands up by your head and slightly in front of you (towards your target)to act as a counterbalance as you lean over during the kick. Additionally, they prevent the necessity of blocking with your face. We will be kicking with our left leg. From the middle stance, slide your right foot towards the left until your right heel touches the side of your left foot. Your feet should be in either a T or L shape at this point. Now, your back and butt should be turned toward your target, with your head looking over the left shoulder so you can see what you're hitting. Chamber your left leg by picking it up off the floor and holding it such that the bottom portion of your leg (from the knee to the foot) is parallel to the ground. Imagine that your leg is sitting on a table that is waist high. A line drawn from your knee to your heel should point directly at your target. Execute the kick by extending the leg along that plane until it is fully extended. The heel of your foot is the primary striking surface, with the ball/toes pulled back towards you, and ideally slanted just slightly down toward the ground. Now, re-chamber by pulling the leg back along the exact same path, and set it down under control (eg. don't let it fall to the floor after the kick).
Congratulations, you have executed a side kick!
Side Note: When performing any kick, it is absolutely critical that your leg is straight but not locked. The muscles should be stopping the motion, not the joint itself. Doing otherwise can lead to hyperextension of the knee (which tickles).