If you have a puppy it will be much easier than you think; in fact, the puppies get tired quickly and sit many times a day. Simply reward the puppy with a treat every time he or she sits down, saying the sit command. After a few days the puppy will associate the word sit to the sitting position and the reward.
In the case of an adult dog the process will be different, since in periods of fatigue will tend to lie down rather than sit.
Here is the method:
1) Take a nibble and put yourself in front of your dog
2) Put the treat on his nose and move it in a straight line parallel to the ground towards its tail
3) The dog will be physically forced to sit
4) Reward the dog while saying “sit”