How you install user scripts depends on which browser you use. Typically, you will need to install a browser extension. This is not an exhaustive list, please contact us if you have any additions.
Once user scripts are enabled in your browser, it should be as simple as clicking the "Install" button on any script's page on Greasy Fork.
Opera version 12 and earlier supports user scripts natively. Violentmonkey provides a friendlier UI and better compatibility.