Aspirants can develop subject matter expertise in their optional subject by engaging in a structured study plan, which includes comprehensive reading from standard textbooks, journals, and authors known for their expertise. Joining study groups helps in exchanging ideas and resources, while practicing answer writing refines application skills. Additionally, aspirants should stay updated with current developments related to their subject, integrating contemporary issues into their understanding. Participating in debates or discussions can also enhance analytical skills and knowledge depth, making them experts in their chosen optional subject.