The first step in leash training is getting the puppy used to a collar. It should begin within the first few days after you get your puppy. It's usually better to let the puppy get used to his new surroundings before introducing new items such as a collar. Then a day or two with just soft collar, no slip collars or choke collars at this point. The next logical step to me is to add a length of soft rope or flat lead to the collar ring so that is drags the ground throughout the length of your puppy's body. Each time the puppy steps on the the lead or if it catches under a furniture leg it will help the puppy learn that when something tugs on the lead it means stop. You can do this for several sessions throughout the day but only when you are present, each session should probably last 15 minutes or so . At the same time start taking your puppy on a leash whenever you go outside and also for training walks throughout the house.