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.
With your hips and shoulders rotating, swing your front leg upward and across your body in a circular motion.
The foot your are standing on should naturally rotate to help open the hips and give power to your kick.
Come back to the starting position, either by reversing the motion, or by completing the rotation, 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 shin.
Point your toes to avoid smashing them (in case you hit a real target).