The Tornado Kick is a very complex kick with alot of detail. Many different styles of martial arts have now added this kick to their forms and many of them perform it poorly.
To perform the Tornado Kick, you must take several steps for momentum (usually no more than three). Dip your body by squatting and using your arms swinging them from top to bottom and around to the left hand side. Your left leg should tuck up to your chest (this is where many people perform it wrong, as their knee points to the floor and then tuck the leg back looking like a V shpe from the side).
Once your left leg tucks tightly to your chest, jump off the right leg and kick with the right leg. Slap your right leg in mid air all while still keeping your left leg tucked. Your back must be completely straight (this is also where most martial arts mess up this kick as they always tend to lean back since they lack the abdominal strenght to tuck the leg tightly to the chest).
As your body rotates in the air you come back down with your back straight and land only on the right leg the the left. Once again, leaning back messes up your landing because after you rotate your body, you end up landing face down which could break your knee with all the impact and momentum.
* Tip - the most important thing is to keep your back straight, and use your arms for momentum.