It is nice if your dog walks nicely to heel. I find the best time to train a dog to heel is after some energy has been expelled on a walk. Once your dog has had time to calm down, put your dog on the lead. You must start with your dog on the left hand side. Your dog must not be ahead of you at anytime. With a nice loose lead, walk steadily. If your dog pulls or leaps around beyond your control you must stop and make your dog sit. Once you are walking your dog with relative calm then you correct any surges forward with a quick direct tug of the lead. As demonstrated acurately in the first video link. It is important to think about why your dog might be pulling. If it is at the start of a walk then there is little point trying to train a dog to walk to heel at the start. You may find it more productive to work at the heel training at the end of the walk when the motivation to pull has been removed (ie the desire to get to the walk quicker). You must always stop if the pulling is not effectively controlled by the corrective tug on the lead. It can help to change direction regularly when doing lead work. You use your knee to steer the direction (if necessary). Keep doing a box shape, but changing direction so your dog cannot predict your next move, thus putting you in control. When you have put enough practice in, you can then continue the training at other times of the walk when you have more confidence to put your efforts into working practice.
With the help of my daughter I have taken a video clip of Katherine walking Charlie to heel. The first clip is short, but demonstrates how to correct any pulling effectively by pulling the lead quickly and directly.
This second clip gives you another opportunity to see Charlie walking on the lead. Pulling does not have to be seen as pulling but even one step ahead of you must be corrected.
Katherine then demonstrates with Melvin, he is pulling to get to me (Im in the road) Katherine corrects him every time. It is consistency that is key. So you must correct every time the dog pulls. I think Katherine has made a sterling effort with Melvin given his distraction! (me!).