I accidentally closed the introductory WriteMapper tutorial / guide. How do I open it up again?

You can do this from the menu bar of the app.

Open the WriteMapper app, then navigate to Menu Bar → Help → Open Tutorial WriteMap.

On Windows, the menu bar may be shown by clicking the little menu icon in the top-right hand corner of the WriteMapper app.

On macOS, the menu bar is located along the top edge of your screen, while ensuring the WriteMapper app is in the foreground.