How to import my feeds in my (classic) Merchant Center account?

This FAQ described the former classic Merchant Center interface. Its old menus and screenshots are no longer the procedure for adding a feed.

See How to import product feeds into Google Merchant Center for current source setup and scheduling.

With XML + CRON, retain this order: complete file generation, then Google's fetch with sufficient time in between. Give Google the XML file URL, not the URL triggering the CRON task.

For detailed requirements: Official Google documentation (1), Official Google documentation (2).