To split a large PST file, you start by opening the SysInfo PST Splitter and adding your PST file. The tool displays all the folders inside the file so you can pick what you want to split. You can choose to divide the file based on size limits, choose certain dates, or split by senders or folders. Once you set your preferences, the software creates several smaller PST files and saves them to the location you choose. This helps you manage Outlook data more easily and avoids issues caused by oversized PST files.