When in striking stance it is important to keep your shoulders up in order to protect yourself. If you lead with your shoulders up, incoming punches will be deflected away from your jaw, which is the "knock-out spot".
Step 1: Get in striking stance
Step 2: Lead with the shoulder on the same side as your lead foot.
Step 3: Keep lead shoulder higher in order to protect yourself, but don't neglect the weak side shoulder.