Leash training your puppy shouldn't just start when you're ready to take your dog out for its first walk after it has all its shots. Leash training should start in the house. A dog should get used to the idea of being restrained, the dog should get comfortable with the leash.
Now there's a couple of ways you can do this. I've done where you just let the dog drag the leash behind, obviously supervised to keep it safe, but let him drag the leash behind him and it doesn't matter if he's chewing it or he's grabbing at it, let him just used to the sensation of having it on there.
You can pick up the end of the leash and walk around with him. Just kind of just follow him around your home with the leash on just so he gets used to that idea.
What we can then start doing is introduce reward into that behavior so that he's walking on the leash and he starts focusing on us for reward. That sets him up already before he ever goes out for a walk where there's going to be a million distractions. He's starting in your home, minimal distraction, having rewards for walking next to you on a slack leash which is essentially what we want in the long run.
Once he has the blue print for that attaching it to the outside world is going to be much easier for him. So he can drag it around behind him.
You can also, some people I've worked with, it's very clever, they'll hang just the buckle of the leash, just let it hang on there so they get used to the sensation of something on their collar or on their harness.
Getting the dog comfortable with all these things before it's actually out in the world, don't just put him out there and start punishing him for making mistakes he didn't know were mistakes in the first place.
When a dog goes outside for the first time he's going to be really distracted and really excited to see the world around him and he should be. We want him to have that pleasant and exciting experience. So punishing him for pulling or dragging behind is really making the leash walking less pleasant for him and can have adverse effects on us later on.
So it should be something that he finds really pleasant and he's really used to before you ever go out in the first place. And that's how you leash train your puppy.