Now click the settings/cog icon in the top right-hand corner, then click the configure this page item
Now go to the advanced tab
Now click "Save" or "Save and reload the page". The page needs to be reloaded for the script to run.
If you need any corresponding HTML on the page for the scripts to run against then use the page editor with the code icon on the toolbars </>
If you also need to add custom CSS styles then enter those in the Custom CSS styles box. You DO NOT need to add style tags here, just the custom CSS. For more info on adding custom styles see this guide