Stand up and bring your guard up (elbows bent and close to your body, hands in front of your face).
Stagger your legs to have your working leg at the back.
Pivot on your front foot to turn your side toward the target, and bring the working knee up toward your chest, as if you were preloading a spring.
Explosively extend your leg sideways toward the target, driving your heel in a straight line.
At the same time, keep rotating the standing foot so that it points away from the target at the moment of "impact", letting your hips turn over. That twist is what gives the power to your kick.
Bring the knee back, put the foot down in your stance and repeat, then switch to the other side.
Form tips
Imagine a target to your side that you have to push away with your heel.
At the moment of "impact", your hitting foot should be horizontal and your heel, hip and shoulder in line.
