After you've set up the extension - go to the page you want to annotate
Press the extension button (or use the
Alt+Shift+H shortcut) to activate it on the page
On the first usage you'd be required to log in with your matrix chat account.
To start annotating - you need to select which Matrix Room will your annotations go to.
You'll be prompted to do that when you try to create your first annotation.
Or you can opt in to join an existing discussion room in which case you'd be asked for a room id.