First, you need a userscript management extension. If you try to install userscripts before installing the extension, it won't work. If you're using Chrome, install Tampermonkey - Chrome Web Store. If you're using Firefox, install Greasemonkey - Add-Ons for Firefox. There are also some options for alternate browsers - see Which Browsers Can You Run Userscripts In, and How?. (The vast majority of turkers use Chrome and/or Firefox.) Then, go to the Turking Scripts set on Greasyfork and find the userscript you want to install. Be selective about what you install (there are hundreds of them), and read the info on their Greasyfork pages to understand what they do. Different workers prefer different combinations of scripts to fit their personal preferences. You might need to adjust the execution order for some combinations of userscripts.