There are two ways to insert the diagram into the Confluence page
From the toolbar, click at insert icon (+), then select Mermaid Diagram from the search result
Using the slash(/) shortcut, type /mermaid in the Confluence Page, then select Mermaid Diagram from the drop-up
When done, the diagram looks like. You can click in the Edit (pencil) button for configuring the diagram.
If you see the box below
You will need to publish or preview the page from the top right of the toolbar
Then click at Allow button, before you start to use it.
Next steps:
How to update an existing diagram?
Prepare for exporting