First Teach your dog to lay down with a treat lure as shown in this video. the next step will be dis-associating the treat from the command/request. After you’ve done many many repetitions of the action, try inserting the word cue just BEFORE you perform the luring action (repeat many times). Next, pause in between the word and the luring action. Watch your dog processing! He is trying to figure out if you will put down that treat next. If you wait a little he may just lay down trying to get you to hurry up! If that happens, boom! your dog is a genius! Lavish treat rewards!
Next Step:
You will want to work your way into an upright position from which you ask for “down” with the word cue . Don’t worry this takes a little time, but it’s necessary for your dog to put it together that he can respond to the word and not just follow the snack!