The STAR method is a framework for answering any kind of question that requires you to tell a story. It will keep you focused on what's important, make sure that you cover all of the key details, and give your answer a reliable, tried and tested structure. The STAR method consists of 4 stages.
Situation: Give your story some context. It's important that your interviewer knows the background for your story. But they also don't need to many details. Just enough for them to understand what is going on. A common mistake is to spend too much time on this stage.
Task: What did you need to achieve? You might think this is obvious if you've explained the situation, however over communicating can be really effecting in job interviews. Make sure that your interviewer knows what you were trying to achieve before you start to give details of how you tried to achieve it.
Action: What steps did you take to achieve your goal? This is the main part of your story. Talk about what you did and feel free to go into detail. Breaking your actions down into numbered points can help you and your interviewer to follow the story.
Result: What happened in the end? Again, you may think that this is implied by the rest of the story, but giving your story a nice and neat conclusion can help your interviewer to understand exactly what you want them to take away from this story. What do you want them to write down and remember? Why were your actions important?