Put your hand with the treat to your dogs nose, allow your dog to lick the treat and slowly lift the treat above your dog's head and back a little. Your dog's nose should follow your hand up and move your dog's butt down to the floor. Once it's butt hits the floor, say "Yes! Sit" and give your dog a treat. Repeat until you no longer have to put a treat to your dog's nose and it starts offering the sit on it's own.