Step 1: With the dog
in a down position, place a treat in your hand and lure the dogs' head until it is touching the ground. You may need
to move the treat away from the dogs' body so as to get them to lower their head instead of just reaching down for the treat.
Say "head down".
Step 2: Click and reward when the dogs' head gets close to the
Step 3: Continue doing this, gradually requiring the dog to lay its head
closer to the ground before getting a treat. Eventually you can require that the dog lay its head down and leave it there
for a while before treating.
Step 4: Once they get the idea of "head down", you
can give the command while the dog is standing or sitting, tapping your lap, a chair, the bed, etc. and asking the dog to
put its "head down" on that object.
The above info is provided by Gio FM.