It's simple.
- With a calm assertive voice, say "Sit" one time.
- Slowly push his lower back down into the sitting position.
- Say "Good boy!" and give him the treat.
Refrains:
Do not be repetitive. (The command is "Sit", not "Sit Sit Siiiit Sit! Sit")
Do not say "Sit" as if you were asking a question. (Always speak like you are the Master)
Do not give your dog the treat if he does not "Sit". (This reinforces incorrect behavior)