Start out by using the harness in a calm / low distraction environment; this is new for both of you.
With especially energetic dogs, use this dual connection until you both feel comfortable with walking calmly.
Start your walk with a "short" or a "tight" leash, without slack. Keep it taut, but don't tug!
For "good dog" walks, the leash must to be short and taut. For pooping or "at ease" walks, let the leash out.