Terriers frequently require different training techniques than other dogs, but what I'm about to suggest will work for any dog. This is how we are training GG, our foster Airedale, to sit.
The easiest and quickest way to teach "sit" is to let your adoptee see other dogs doing it for a reason. This is often called putting the behavior in context. The best reason for a dog to sit is so that he can receive a treat. If you have another dog that already knows the sit command, you won’t have a problem teaching it to the new dog in your family. If you don’t have another dog, find someone at a local park who does (*not a dog park!) or get a friend who does have such a savvy dog to help you. And take a training partner with you who can hold your dog’s leash while you are working with the other dog.
You’ll be using treats and hotdog wieners are great for this project. Space the dogs about eight feet from each other so there won’t be any difficulty with treat possessiveness. Have your partner hold your dog’s leash. Then give each dog a small piece of the treat without asking either one to work for the reward. The next time, go to the other dog and tell her “SIT!” When she does, give her a treat and praise her with “Good SIT!” Be sure to put the emphasis on the command.
Repeat this three or four times. Then return to your own dog and let him see and smell a treat in your hand. Do not say anything. Just hold the treat out where your dog can see it and wait patiently for the canine light bulb equivalent to turn on in his head. If that doesn’t happen within a couple of minutes, go back to the trained dog and repeat the Sit exercise with her. Then return to your own dog and try it again. This technique works fairly quickly because hotdog wieners are a smelly, moist, tasty incentive. When your dog does give you his first Sit for a treat, praise him in a quiet voice. The second time he does it, praise in a louder voice. The third time he does it, praise him as though he just walked on the moon.
Now that your dog has the basic understanding of the command being communicated to him, practice “SIT” in every room of the house. It’s important for Fideaux to understand that "sit" means sit no matter where you say it.
* Save the dog park experience for the time when your dog is completely successful with his basic obedience skills. He'll be more confident and less likely to be reactive if a negative situation arises.