There are a few different ways that you can train your dog to be aggressive towards strangers. One way is to expose them to as many different people as possible, so that they become used to seeing new faces and aren't as easily startled by them. You can also use positive reinforcement training techniques to teach your dog that strangers are not something to be afraid of. Finally, you can socialize your dog with other dogs who are known to be friendly, so that they learn how to interact with new people in a positive way. Whichever method you choose, it's important to be patient and consistent with your training so that your dog can learn the behavior effectively.