All you need to do is hold a treat or something your dog really likes, like a bone or tennis ball just above their nose and move it towards the back of their head. By doing this motion it will force your dog to sit to keep an eye on the prize. Make sure the treat is not too high or too much on the side and do not go too quickly.

You may want to try a few times to see how your dog reacts, once you think you got the height and motion down, start using the command like “Sit” during the motion and your dog starts to sit. Once he is sitting give him the treat and reward him with praise and say “Good boy”.