To manually convert Office 365 to MBOX, first export Office 365 emails to PST using Outlook or eDiscovery, then use a PST to MBOX converter tool like Thunderbird with the ImportExportTools add-on, and finally save the converted files in MBOX format. However, the manual process is time-consuming, prone to data loss, and requires technical expertise. For a more efficient and error-free solution, consider using a professional tool like DRS Office 365 to MBOX Converter to automate the conversion process seamlessly.