Is there a guided walkthrough for setting up a tournament?

Yes. The first time you open a new draft tournament, a short spotlight tour highlights the key stops: your dashboard, the publish checklist, Divisions, Settings, and Branding (where the required header image lives).

To take it again, click "Take a tour" on a draft tournament's dashboard. You can turn the guided tours off (or back on) under Settings → Guided tours.

The tour runs on the web app on a desktop-sized screen — on a phone or narrow window it steps aside, since the sidebar it points to is tucked away.