Once your dog can stay in a sit for several seconds, you can begin adding distance. Place him in a sit and say “stay,” take one step back, then step back to the pup, give a treat, and your release word. Continue building in steps, keeping it easy enough that your dog can stay successful. Practice both facing him and walking away with your back turned (which is more realistic).
Just as a child needs a caring parent; an athletic team needs a coach; your puppy needs a leader and a clear social hierarchy. If you do not take up the role of leader, your dog will; and you will end up with an unruly, disobedient dog. Many people try to win their new puppy's love by letting the puppy always have its way. Buckets of affection is a wonderful thing for most puppies, but it must be tempered with respect.