I teach with my dogs on my left side but it can be taught on either side. Start just standing still, pull straight back slowly on the leash until there is a good amount of tension on it. Hold tension and as soon as any of your dogs feet make a movement backwards mark the behavior and immediately release the pressure and reward. I use the word yes for my reward marker but it can be anything you want. Something quick and easy to say, timing is the key. I also teach leash pressure in all directions, side to side and forward, it will allow subtle adjustments with the leash while teaching heeling to keep the dog in perfect heel position. Once the dog is moving easily with pressure you can start giving the pressure a command, like back, say back first then apply the pressure. Eventually your dog will back up with just the command and no leash pressure. Train all steps before taking it outside with distractions.