To prepare an inground swimming pool for winter closure, start by thoroughly cleaning the pool, removing any debris and dirt. Next, balance the water chemistry by adjusting the pH level and ensuring proper sanitizer levels. Shock the pool with an appropriate treatment to eliminate bacteria and algae. Clean and backwash the pool filter, emptying pump and skimmer baskets. Finally, lower the water level below the skimmer and return lines, and install a secure winter cover to protect the pool from debris.