After installing UpFish, you should pin the extension so that it is visible.
You will need streaming access to the movies you want to modify. Any streaming service is acceptable. If the service injects server-side ads, sync may be lost, so purchased copies of a movie are preferable.
After loading the movie in a tab, click on the extension and select a config.
When the extension is active on the current tab, the icon will be salmon-colored.
Navigating within a tab will deactivate the extension on that tab.