If you're talking about the famous Muay Thai round kick, the rear thay (sp?). here's the little I know.
Lead with the hip. Turn you hips, and body to the point that your suporting foot turns (on the balls), so that the heal almost points forward.
If you are left lead, your supporting heal should point to about 2 o'clock.
At impact, I think your hips should point to 12 o'clock (I'm not so sure of this).
Keep your kicking knee partially bent. DO NOT SNAP. This kick slams in like a baseball bat (not like a hammer). You strike with your lower shin, not the instep. If you miss, you usually spin around. When you do hit (a bag or a person), you can usually get back to onguard by straightening the leg, to push off the target.
Practice on a heavy bag.
My $0.02, from JKDC training (not actual MT, so what do I know)