Chuck-it type ball (easier for puppies) OR other favorite toy
STEP 1: Have your puppy near you on the long line. You can have the line short at this point to keep them near you.
STEP 2: Throw the ball as you say "get it".
STEP 3: (Super important for the bringing back) Wait until your puppy latches onto the ball before saying "let's go". DO NOT say "good boy" or "let's go" until they have a grip on the ball.
NOTE: If they don't go after the ball, first make sure you have not picked out a time the puppy is tired or has already trained enough. Remember you want to do this when the puppy is fresh and ready to go. If they are, but something has just distracted them or inhibited them, go towards the ball yourself with the puppy. They will normally go and grip it after this.
STEP 5: Now it is time to get the ball. You don't want to make a big deal of it where the dog is inhibited. Just nonchalantly say out, and hopefully the puppy does not give up to much of a fight. If they do, I just pet the dog beside me and hang out and wait for them to drop the ball. So the point is, you don't want the puppy to get the idea that he is going to be forced to give up the ball with undue force at this time, as in the beginning you want the puppy to be "up" for the game. NOTE: Timing is crucial here. Throw the ball right away (easily for young puppies not too far or fast).
STEP 6: Repeat. Notice if the puppy gets tired of the game after some reps, because you do not want to burn them out on this. It is better if the puppy still wants the ball and to engage when you stop, rather them tired at the end. I would do no more than maybe 10 or 15 reps. Depending on your puppy though, it could be even less.