Our experience - maybe this can help a little. Ours is also "pet trained." I honestly don't remember how we did our first leash training - maybe our pup took to it easily? But training to put on the harness has been interesting. I learned from my husband, the less you try to control the dog and the more you just go with the dog's movement and slip on the harness the better. Finding the right harness has been the biggest challenge. I am now crocheting a harness that won't rub her tender skin. She is 2.5 years old now and there are always new lessons to be learned. She goes for our exercise walks and needs to keep pace, nose up.
Make sure your leash is long enough to hold in one hand, cross over your body to drape over the other hand and down to the dog. Make sure the harness fits properly and there is no pulling on the neck/trachea. (No leashes attached to collars.) The taller you are the longer the leash you will need. We found with a previous dog that the leashes you can let out and retract in just confuse the training and teach the dog to pull to signal it wants a longer leash. I'd avoid those.
She's had to perfect loose leash, no pulling walking. It takes a few steps every walk to get her in rhythm. The best tip I found was if the dog is pulling - STOP! If you allow the pulling, they get what they want. So, stop, recompose, and then continue. We try to give her some nose down time on breaks or at the end almost as a reward. I have found "exercise finish" a better reward for her than treats as she's older. She's much too focused on the treats to perform her tasks well if we go that route. So, a lot of love and for her, the satisfaction of pleasing us are now (that she's older) the best bet.
Keep your initial trainings and walks short to match the pup's age and attention span. It will get longer.
