Focus for Heeling
This is how I start it with all dogs, pup or adult, trained or untrained. I do modify it as I progress and see either strong progress or issues.
Each step is started in a neutral environment, as the dog progresses in that step distractions are slowly added. I hold my hands away from my sides (like a T) with a treat in each hand, with the dog sitting in front of me. Doesn't matter if he sits or stands, but I start in a sit to lesson the jumping around that may initially happen. I do the T position so it makes it VERY black and white to the dog, and me, when they are looking at the food vs my eyes. If your hands are at your side, or in front of you, it's not quite as black/white. The dog will probably nudge your hands. He may stare at your hands. He may jump up on you, circle, you, etc. However, eventually he will look up at you. When he does mark this behavior with a single word. I use "Yes". At first the eye contact will be fast so be ready. Then reward him after the "Yes". Reward one time from your left hand, one time from your right hand, etc. Keep it random so the dog never knows where the reward is going to come from. This will take lots of repetitions, but it will get the dog to understand to focus on YOU not the food.
Once the dog is making solid eye contact with me for 5-10 seconds, I start to add some MINOR distractions. This can be as simple as me slowly moving my hands around. Generally the dog will glance away, I say "uh-uh" and when they look back I mark it and reward. As they understand the concept, increase the distractions. Move your hands faster, move them behind/over the dogs head, have someone else walk around, clap their hands, etc.
Once the dog is solid on this, then I put them into heel position and basically start from square one. Have them focus in both a sit and a stand position, but you are not taking any steps. I will introduce some SMALL corrections at this point also for looking away. Once the dog can focus in both sit and stand next to you, with distractions, then I will start to move. I'm not walking multiple steps yet, just one step at a time. And in all directions, ie 1 step foward, one step backwards, one to the right, pivot 1/4 turn on the right foot (turn to your right), pivot 1/4 turn on the left foot (pivot to your left), etc. This teaches the dog how to move it's body around in heel position without loosing focus. If need be I will lure the dog at first to help them with the movements, but fade the lure as quickly as possible before it becomes a crutch. Once they are solid with this, then I will start to walk. At this point I like to be holding the food in my left hand, behind the dogs head. If they really understand focus they won't be dinking around trying to get the food out of the hand near them, or turning their head, they will be making eye contact. Even with the "single step" exercises, the biggest issue I've had is that first step as you heel, some dogs will want to drop their head and break focus on the first step, then resume focus on step two. So look out for that. If need be, use the food as a lure to help them transition from sit to walk without dropping their head. Do lots of start/stops to practice the transition without breaking focus. Once the focus is solid, bring back the distractions.
Once I am no longer working start/stop and really short heeling (5 or fewer steps), I transition from the food to the toy. If the dog has higher toy drive. If their food drive is higher, or the drives are equal, I will continue with food. I always start with food because of the ease of reward, you pop something into their mouth, they swallow and are ready to work some more. They don't even have to get up. A toy takes longer by the time you reward, play, take the toy away, get the dog back into position, and start again you could have done 3 more reps with the food.
Keep in mind, the focus work is actually easier for the dog if the reward is "behind" them. Behind them, off to one side, whatever. If it's directly in line with their head and your face the temptation to flick their focus back and forth from you to the toy is very great, and your timing will be off on the marking of the behavior. I do a LOT of heeling around the decoy with Mac, and he gets to bite when I get 110% of his focus. I almost always heel counter clockwise, because that puts the decoy to Mac's left, and he has to turn his head to the right to look at me. But we have been doing this for 10 months now, and have progressed so now we are also approaching the decoy head on, turning to the right, working with multiple decoys on the field, etc.
I have two commands, "look" and "heel". This way I can tell the dog what they are doing wrong. But I would also blend this for competition purposes, so they understand heel means look. You will still have the seperate look command, which is useful if the dog is doing a long down and not paying attention, they understand "look" doesn't mean come to my side but it does mean quick looking around and look at me.