For a project question, start with the problem and your responsibility. Explain the decision you made, the tools or methods you used, and the result. If the result was incomplete, describe what you learned and what you would change.
For a behavioural question, organise your answer around the situation, task, action, and result. Keep most of your explanation on the action you personally took. Avoid inventing dramatic outcomes when a modest, well-explained example shows the skill.
For an unfamiliar problem, clarify the assumptions, reason through an approach, and identify what you would need to check. Acknowledge uncertainty directly rather than filling the gap with a confident guess.