Candidates should approach the optional subject in a past UPSC exam by conducting thorough research on the syllabus, previous years' papers, and trends in question patterns. Selecting an optional subject should be based on personal interest, academic background, and potential scoring. Focused preparation can involve analyzing, synthesizing, and applying core concepts, alongside regular practice of answer writing. It is crucial to integrate knowledge from related subjects and ensure a depth of understanding, as this will enhance the quality of answers during the examination.