Thumb is wrapped around fist. Fist is held against rib cage. Elbow is directly back, not winging out to the side. Shoulder down, not tense. This is a basic punch.
Hips provide momentum and power to punch.
Your hips can control your entire body weight. Put it into your punch. Forarm rubs against your side as your fist moves forward. Prevents winging of the elbow. Provides extra power when friction ends.