To get userscripts going with the desktop version of Tampermonkey, first you have to install it from Apple Safari Extensions. From the webpage, click the text "Install". Upon completion a Tampermonkey icon should appear next to the address bar.
Tampermonkey is one of the most popular browser extensions across all major browsers. It's available for Chrome, Microsoft Edge, Safari, Opera Next, and Firefox. It allows its users to customize and enhance the functionality of your favorite web pages