In clicker you never put the word before you have taught the action and they offer the behaviour. Dogs do not come knowing that down means belly on the ground so you teach it first then name it.
Another way to do it is to sit on the ground. Legs out straight. raise one leg. Lure dog under the raiseed leg and as the belly hits the ground under the lag click and treat. that is actually lure training not cliker but it works.
Actul clicker would be to shape the down with out any comment at all. And if you youtube look up Miranda Bouque and she teaches a clicker down to izzy.
JL