Tips for Training Your Dog to Sit
According to Dr. Nicholas Dodman, put your dog on a leash and attract his attention so he looks directly at you and you at him ("Watch-me"). Then issue an action word, SIT. A poorly trained dog might slowly get into the sitting position, at which point you reward him IMMEDIATELY with praise, GOOD BOY, ROVER, (remember the high tones and heartfelt deliverance) and at the same time as you immediately produce the reward.
For more information – please read How to Teach Your Dog to "Sit".