I started a thread about this below to get people's thoughts.
You'll need a partner to do this, but this is how I would teach it. Mind you, I've never taught it so this one is off the cuff. Chris probably has a better method.
Have your partner stand with the food reward or ball or tug or whatever. Make sure the dog is stimulated to focus on the reward. Put him in a sit. Next, get into the backwards heel position (face the opposite direction from the dog with him on your right side) and deliver the backwards heel command. Then verbally release the dog to take the reward and praise. Repeat this for the next few lessons.
I would then proceed by setting your partner up the same way, putting your dog in a sit in proper heel (left side) and then delivering the backwards heel command. If you need to guide the dog into position by pulling the leash in front of you and have your partner stimulate him a little with the reward then go ahead. Just make sure the dog is orienting his position to your right leg.
Once the dog will move into the backwards heel starting position reliably I would go to the next step.
Now take one step at a time forward while your partner walks lock-step with you. I wouldn't worry about auto-sits on the stop because ultimately the dog is going to be moving with you the whole time anyway. Just make sure that you don't release and reward the dog unless he is in perfect position. Ensure that your partner works to hold the dog's attention without enticing him to move out of position (have him jiggle the tug a little). You may want to implement leash corrections at this point.
Now it's just a matter of perfect, consistent repetition.
Again, this is how I THINK I would train this particular behavior. Having never done it I don't know if this is the standard way.
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