Training method: Treat
The sign for “down” is going to be just like the sign for “sit”, but instead of lifting your hand up, you will lower it down.
Start with your dog in a sitting position. Hold a treat between your thumb and index finger, and try to keep your palm as flat as possible. Hold your hand above the dog’s head, and slowly lower your hand, palm facing down, to the ground. Your pup should follow the scent and lower himself into position. Make sure your hand is close enough to the dog’s body that he doesn’t need to move forward to reach the treat. If he doesn’t follow right away, leave your hand lowered for a minute. He will probably sniff and paw at your hand, but will eventually lower himself to the ground to try to get to the treat. When he does, reward him immediately with the treat and the “good dog” sign. If he stands up, redirect him into the sitting position and try again.
Here is a little video of my former foster puppy, Abe, mastering the “down” sign. You will see that we are no longer using a treat as a lure, but he is following the sign. He first tries to paw at my hand, but then remembers his cue and gets his reward! Good job, Abe!