Once your puppy has mastered “sit” it’s time to move on to “down”.
Begin by having your puppy sit.
Hold a treat in front of his nose. slowly move the treat down towards his toes.
If he follows the treat, when he’s close to being down, command “down”.
Praise with a “yes” and reward with a treat.
If he needs a little more direction, start again and move the treat more slowly.
If even more direction is needed, try sitting on the floor beside him. Hold the treat under your leg so he is forced to lie down to get the treat. As he goes down on his tummy say “down” and reward.
With every training session, give the command as the behavior is accomplished, followed by immediate praise and reward.
As the behavior is learned begin giving the command before the puppy does his trick.