Not sure if this is a Taekwondo side kick, but I'll try to describe how I personally perform a side-kick.
1. Spot my target. Preferable a low target, such as the groin or the knees; this will deliver maximum damage with minimum effort, while at the same time protecting my leg from an easy grab and take down on the behalf of my opponent.
2. Staring front on at my opponent will be called "12 O'Clock" for descriptive purposes; I lower my body profile, centering it over my hips, keeping my solar plexus aligned over top of them; the body's center point of gravity. This provides me a strong foundation, and protects me from being pushed over too easily. In the same movement, I pivot with my strong leg towards 6 O'Clock, with my dominant foot pointed closer to 5 O'Clock for better balance. At the same time, I'm pulling my non-dominant knee up towards my chest. My upper body is now horizontal to the floor, and my weight is balanced on my dominant leg, with the other leg poised to strike.
3. Using my arms to help maintain balance, I now take my eyes/peripheral vision off of my target site for a split second, visualizing a straight line from 12 O'Clock to 6 O'Clock, moving my head towards 6 O'Clock and elongating my torso while snapping my raised leg out straight along the imaginary line to my target site, driving with my hips like a swivel and moving my arm to the rear to give the kick more momentum.
4. Immediately recoiling my kick, I spin around the opposite direction that I originally pivoted, while simultaneously raising my arm up and wrapping my hand around the back of my head, flexing the muscles in my arm and assuming a lower, tighter profile. This will protect me in the event my opponent is unaffected by the devastating kick I just delivered, and comes at me with an immediate strike to my face or torso. Chances are my opponent will either be on the floor (if I took out their knee cap) or hunched over in a "tummy ache position" (if I took out their nether-regions). Both positions are ideal for me to immediately follow up with a finishing move to destroy my opponent, assuring that they won't be able to pose any more of a threat to me; either a ground and pound, or downward elbow strikes to the back of their head if they've assumed the tummy-ache position, then a supplex through the nearest wall.