The approach to studying for the optional subject in UPSC Mains should involve a thorough understanding of the subject's syllabus, followed by focused reading of core texts and additional materials. Candidates should make concise notes during their studies, allowing easy review closer to the exam. Practicing answer writing on previous years' questions is crucial, as it enhances writing skills and identifies areas needing improvement. Engaging in discussions or study groups provides multiple perspectives, enriching understanding. Lastly, balance between the optional subjects and general studies ensures comprehensive coverage of the entire syllabus.