Introduction:
To ensure a deep understanding of both political theory and international relations within the PSIR optional subject, you can follow the below strategies:

1. Comprehensive Reading:
Read extensively on both political theory and international relations. Develop a habit of reading books, research papers, journal articles, and other relevant materials. This will help you gain a strong foundational knowledge and a broad understanding of the subject matter.

2. Analyze and Evaluate:
While reading, make it a habit to critically analyze and evaluate the theories, concepts, and arguments presented. Try to identify the strengths, weaknesses, and limitations of each theory or approach. This will help you develop a nuanced understanding and a critical perspective on the subject.

3. Make Connections:
Try to connect the concepts and theories of political theory with international relations. Understand how the two fields intersect and influence each other. This will enable you to develop a holistic understanding of the subject and its multidimensional nature.

4. Historical Context:
Study the historical context of political theory and international relations. Learn about the major events, movements, and ideologies that have shaped these fields. Understanding the historical context will provide a deeper insight into the theories and concepts being discussed.

5. Comparative Analysis:
Engage in comparative analysis of different political theories and approaches. Compare the perspectives of different scholars and schools of thought within political theory and international relations. This will help you understand the variations in theories and develop a comprehensive understanding of the subject.

6. Current Affairs:
Stay updated with current affairs, especially those related to international relations. Understand how political theories are applied in real-world scenarios and analyze the implications of international events. This will help you link theoretical concepts with practical situations.

7. Practice Answer Writing:
Regularly practice answer writing to develop clarity of thought and expression. Write essays and answer questions related to political theory and international relations. This will help you articulate your understanding effectively and develop the required analytical and writing skills.

Conclusion:
By following these strategies, you can ensure a deep understanding of both political theory and international relations within the PSIR optional subject. Remember to approach the subject with curiosity, critical thinking, and a willingness to engage with diverse perspectives. Regular practice and consistent efforts will help you master these subjects and excel in your UPSC preparation.