The leash is ready, but your puppy ducks when the harness comes out. You may be tempted to move faster so the walk can begin. Instead, split the process apart. A harness has a shape to approach, an opening near the head, straps that touch the body and a buckle that makes a noise. Your puppy may be comfortable with some parts and unsure about others.
Dogs Trust recommends gradual, reward-based harness training. The goal is a puppy who can approach and wear it comfortably, not one who gives up resisting while you hold them still. Check fit too: a poorly fitting harness can rub, restrict movement or allow escape.
Start without putting it on
Place the harness on the floor during a calm moment. Reward your puppy for looking at it or sniffing it. Let them move away. Pick it up, put it down, reward again. If they retreat as soon as you lift it, hold it lower or farther away next time. You can practise for seconds at a time and stop while things are going well.
When the puppy chooses to approach, hold the harness so they can move their head toward the opening for a treat. Do not pull it over a head that is moving backward. Practise that movement without fastening anything. Depending on the design, you may be able to train stepping into the harness instead. Use the actual movements your harness requires, one at a time.
Next, let a strap touch the shoulder for a moment, reward and remove it. Practise the click of the buckle away from the body before you close it around the dog. The clip can catch fur or skin if you rush. Once all the parts are familiar, fasten it briefly and reward, then take it off. Build to wearing it for a few minutes indoors while supervised, with room to move and rest.
Check comfort before adding the leash
Look at how your puppy walks, sits and turns in the harness. Check for rubbing behind the legs, pressure on the neck or shoulders and looseness that could let them back out. Puppies grow quickly, so fit should be checked often. If your puppy freezes or tries to remove it, don't interpret that as a need for more exposure. Make the step easier and inspect the fit. A sudden reaction after previously comfortable use may signal pain and deserves a veterinary check.
Only then attach the leash and practise a few easy steps indoors or in a safe area. Do not let the first fully dressed experience be a long, noisy outing. If a necessary toilet trip comes before training is complete, use a safe alternative appropriate to your puppy rather than forcing a panicked rehearsal. Ask your vet or a qualified trainer for help choosing equipment if there is an escape risk.
What usually goes wrong
Luring the puppy through the opening with food and then surprising them with a buckle can make them wary of treats in that context. Chasing them around the room with the harness turns it into a game of avoidance. Keeping it on for hours after the first successful fastening may lead to rubbing or distress. Progress in one part does not mean every part is ready. Separate the pieces again if you need to.
If the puppy is very fearful, freezes at the sight of the harness, or shows a new painful response, pause and seek vet or qualified behavior guidance. Do not use a walk as proof that the puppy "got over it" if their body remains tense.
Today's lesson can be one voluntary sniff and a treat. The walk is easier later because the harness did not have to be a fight today. For more on timing rewards during everyday handling, see the dog-handling and training guide, the harness should still fit and your puppy should be free to pause
Sources: Dogs Trust, "How to teach your dog to wear a harness": https://www.dogstrust.org.uk/dog-advice/training/outdoors/harness-training.