To train a dog to sit, you can use positive reinforcement techniques such as giving treats or verbal praise when the dog successfully sits on command. Start by holding a treat close to your dog's nose, then move your hand up which will make the dog's head follow the treat and their bottom will lower naturally. Once the dog is in a sitting position, say the command "sit" and give them the treat. Repeat this process several times until the dog associates the action of sitting with the command and the treat. Be patient and consistent in your training to help your dog learn to sit reliably.