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.
Bring the back leg in close to your body and bend it, as if you were preloading a spring.
Explosively extend your leg directly in front of you, driving your heel forward in a straight line.
At the same time, rotate the foot you are standing on so that it is pointing back at the moment of "impact". Let your hips twist at the same time as you kick. That twist coming from the lower body is what gives the power to your kick.
Come back to the starting position and repeat for as many repetitions as desired, then switch to the other side.
Form tips
Imagine a target in front of you, that you would have to hit with your heel to send it away from you.
At the moment of "impact", your hitting foot should be horizontal.