You start by raising the leg, sort of like you are getting on a bicycle. Then you whip the leg parallel to the ground. Pull it back and set it down. Target is the side of the leg or the ribs.
a roundhouse is a kick that comes "around" and strikes sideways. A sideways front kick if you will, where a front kick is a groin shot. To do a roundhouse kick, you bring get in your stance. Raise up...