Be in a great state, feel good and be confident and your "good body language" will manifest.
How you feel is expressed through your body. And vice versa.
When you're nervous and have anxiety, you have different body language than when you feel confident and are comfortable.
BUT to give you an idea - good posture, stand tall, good eye contact, smile, breathing fully, hands at the side, moving at a normal pace, being present and grounded, chest out, etc...