Create an opening
Control the fight so your opponent is following you. Then guide the opponent into position for your attack.
For example, get your opponent to follow you in a circular motion by moving toward their blind side and then back to the open side. Most defensive fighters will try to follow you. Then take them around to the blind side 90 degrees or better and switch back to the open side while you switch your feet to change the lead. Without warning or even dropping your eyes, fire your lead foot into the mid-section. If you switched quick enough and fired your kick without hesitation, you should catch the opponent trying to catch up to your change in direction.
Most people tend to lead with their upper body on a directional change instead of leading with their feet. They turn their shoulder and, for a split second, they give you an unusually large target in the mid-section because their hips are still facing towards the blind side.