Once Zotero re-opens, go to the Tools tab once more. Select Notero Preferences. Copy and past the Integration Token Key, which you'll find on the My Integration page within your Notion browser. Also add in the database ID that belongs to the No...
In Zotero, Notero Preferences is where I paste the two crucial bits of information: my Notion Integration Token Key and my database ID. Once those are in, the whole Zotero-to-Notion connection actually becomes real rather than just theoretical setup steps.