I would say get it sorted completely with lead on, when the dog is doing it right a small vocal praise. then in an narrow area with no scent or distractions about release the lead and keeping saying heel when dog is doing it. then you can use a stick as a barrier when walking keep slowly swinging it in front of the dog keeping it back. I always get my dogs to heel to the left and use a tap on left thigh to instead of having to say heel all the time.