Scene Notes is operated through a Scene Overlay. To open it, either hover on the Scene View and press the ` key (the one next to Shift, on a British keyboard), or click on the three vertical dots menu at the top-right corner of the Scene View, and select Overlays Menu:
The Overlays Menu, that allows you to enable and disable overlays
Once the Overlays Menu opens, click on Scene Notes to visualise the overlay. Once the overlay is enabled, you should see it appear somewhere in your Scene View:
The Scene Notes overlay, closed
The overlay appears in its closed state. Click on it to enable it, and visualise the Scene Notes:
Scene Notes enabled, with a note visible in the scene
There will be no icons in the scene at first because on the first setup you haven't created any note yet.
To hide the notes again, disable the overlay by clicking on the first button.
Loading notes for a specific Unity scene
Once a scene is associated to a note, that note will be automatically loaded in Unity when the scene is open. Opening and closing scenes will automatically show/hide the related notes.
Focusing on scene notes
If the Scene View is very busy with gizmos and icons, it might be hard to see scene notes. To get focus, click on the Focus button. All gizmos and icons for other components will be disabled, putting the spotlight on the notes.
The Focus Mode button
Click on the button again, or disable the overlay, to see other gizmos again.
When enabling Focus mode, Scene Notes will remember which gizmos were already disabled. This way when you exit Focus mode, those gizmos won't be enabled by mistake.