It doesn't matter if you are leash training a puppy or an adult dog; you first want to make sure your dog is comfortable wearing a collar. The ideal collar to start leash training with is a buckle collar made of nylon or leather. You will want to avoid prong or choke collars. The leash you want to use is a leather or nylon flat leash that is four to six feet long. The thickness of the leash will depend on the size of the dog you are leash training.
The first step when starting to leash train is to get the dog or puppy used to the feel of the leash attached to the collar. For very young or small dogs you can begin with a piece of string attached to the collar and then work up to the use of an actual leash. Attach the leash when the dog is active. You can even attach the leash when the dog is eating. If the dog or puppy begins to chew or scratch at the leash, distract them with a toy or a treat. Never yell or hit the dog for doing so. Start the leash training process by leaving the leash attached for a few minutes. Slowly work your way up to more time each session. Remove the leash only when the dog is not reacting to it.
Once the dog is used to having the leash attached you can then begin to pick up the other end of the leash. Start leash training inside by following your dog around the house holding the leash. You want the leash to remain loose. Don't pull the dog by the leash. Most puppies and even adult dogs will naturally follow their owners. You want to praise your dog when they do as you wish.
If the dog pulls on the leash, you should stop walking. It is crucial that you stop moving every time the dog pulls on the leash. Eventually the dog will associate pulling on the leash with no walking any further. When the dog stops pulling on the leash, get the dogs attention and try to get the dog to follow you. After the dog is doing well with leash training indoors, you can go outside and practice there. You want to pick an area that is free of distractions. A good place is a fenced backyard. Leash training should be fun. Make sure to use patience and always end on a positive note.