If you've got lots of books from Kobo then another good way is to use the Obok Calibre plugin, which is part of Apprentice Alf's tools. That is able to import every book - in one operation - from the Kobo Desktop app into Calibre.
So:
- Install Kobo Desktop app
- Run it and log in with your Kobo credentials. Wait for it to download your library.
- Stop the Kobo Desktop app. Run Calibre.
- Run the Obok plugin, select "all books". Add/import into Calibre.