I used a clicker, put the pup on a leash, threw a treat on the floor and when he went for it, I said 'leave it' and tugged the leash to prevent him from getting the treat, clicked and gave him a different treat. When this works, I took the leash off, held one treat in my hand behind my back, put a treat on the floor, said leave it and handed him the treat in my hand- don't let him get the treat on the floor, reward with one in your hand. There are threads with this on video, you'll have to look for it. Drop it is easy- swap a treat for whatever is in his mouth