It always come to a point whether you have really led a project before. If you never handled any project, you can honestly tell the interviewer "I was not requested to lead any project before as my boss assigned me to something else"

if you have completed a project, then you should start by explaining about the objective of the project, what was the plan to reach completion. Then what was the set back you have to face and your action plan to counter these set backs. In the end, you should tell the interviewer what you gain from the completion of the project in terms of experience and monetary factors.