To use ui-diff you only need to run a command from your terminal when you're inside of the root of your project. There are a couple of options to this command, but start out with running this command.
Sometimes you want to see what is actually happening when ui-diff is taking screenshots of your website. This is especially useful when debugging. To have a headful browser start up instead of a headless one, simply run this command instead.
yarn screenshot --headless=false
If you would like to test your website on a different device than a regular computer screen (which is default) there is a parameter for this as well. Here you can choose between a couple of different screen types.
yarn screenshot --size=(mobile|tablet|desktop)