How do I delete the existing tutorial database and create my own custom one?
Regarding deleting your database, please refer to this article:
https://www.ragic.com/intl/en/doc-kb/128/Deleting-your-Ragic-account
After deleting, you could register a new database with another email address.
Or you could just delete all the sheets and entries in current one, and re-create a new database in the same account.