How to enable StopTheScript:
Purchase and download StopTheScript in the App Store.
Open the Extensions tab of the Settings window in Safari.
The Enable in Safari… button in StopTheScript does this automatically.
Otherwise, you can open Safari manually and select Settings… from the Safari menu in the main menu bar.
(Preferences were renamed to Settings on macOS Ventura.)
In the Extensions tab of the Safari Settings window, check the box next to StopTheScript to enable it.
Check the box Allow in Private Browsing to allow StopTheScript in private windows.
You only need to enable StopTheScript once in Safari, and then it will remain enabled permanently. It will remain enabled even after you install new versions of StopTheScript and Safari. If you ever need to disable StopTheScript, just uncheck the box in Safari Settings.
If you're having trouble enabling the extension in Safari because the checkbox doesn't respond to clicks, please see the Apple support document on this subject. For more details, see my article Safari bug: can't enable extensions on Catalina.