Get onto your dog's level and use a treat to get their attention.
Hold a treat near to your dog's nose and slowly raise your arm above there head. As their head follows your hand their bum will lower to the ground.
Once the dog has sat reward it with a treat and praise.
Repeat these steps a few times. After repeating the steps add the command sit in, as the dog sits tell it the command. Your dog should soon get use to the command and follow it without a treat!