Instructions
Put the harness on the puppy. This is usually best done by laying the puppy on its back in your lap. Buckle the neck strap first, the legs, and then do the body strap.
Make sure you have the proper tension on the straps. You don't want it so tight that the puppy is uncomfortable, but you don't want it so loose the puppy can slide it off.
Place the puppy on the floor and play with it for a while to distract it from the harness. You should have a few of its favorite toys or snacks handy for this process. Eventually, the puppy will get used to the harness and will ignore it.
Take the puppy outside and connect the leash to the harness. Now that the puppy is used to the harness, simply teach the him standard leash etiquette.
How to Teach a Puppy About a Harness
New puppies often react badly to leashes, especially when they are attached directly to the puppy's collar. A good alternative to using a standard collar is to use a puppy harness, since a puppy is less likely to get it tangled or caught on something. You will need to get the puppy used to the idea of wearing the harness and having you lead him around on an attached leash.