Pulling is rewarded every time it gets the dog forward. Change that, and the lead goes slack.
Dogs walk faster than we do, and pulling works: every step forward on a tight lead rewards the habit. A harness can be kinder to the neck, but it does not teach the dog anything by itself.
Start indoors or somewhere quiet. Reward often, with a treat next to your leg, whenever the lead is loose. When it tightens, stop and wait, or turn and walk the other way, and carry on as soon as the lead goes slack.
Short training sessions work better than turning every walk into a drill. Use sniffing on a cue as a reward, and a long line in safe places. What makes it stick is that everyone who walks the dog does the same.
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