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.

On desktop versions, simply navigate to Help → Open Tutorial WriteMap in the menu bar, while the WriteMapper app is open, and the tutorial will open once more in a new window or in your existing window, depending on if you already have content in your existing window.

On the iPad and iPhone, tap on the button that says "Open" in the first introductory screen that shows when you first access the WriteMapper app, and that should reveal a "Show Tutorial" option which you can tap to do just that.