There are a few alternatives, but I'm personally using an extension called Userscripts.
Essentially, you install it from the app store, launch it, and then choose a folder in the Files app where you will store the scripts. Any folder will do, but I think a good idea is to choose a folder in e.g. iCloud so that it's easier to manage from a computer if needed.
After that is set up, you need to go to the Greasy fork link and and click install. You will probably just be presented a bunch of code now. After carefully reading through the ~1600 lines of code (kidding, somewhat) you can tap on the extension settings on the left in the toolbar (the puzzle piece icon) and tap on "Userscripts". Sometimes, the toolbar button defaults to opening the reader for some bening reason, whereas you need to tap it again to get to the menu.
You might be prompted to give access to reading the website, and after all that is done there should be a button saying something like "Userscript detected, tap to install". Tap it, follow the on-screen instructions and you should be good to go.
In the future, the script should update automatically by tapping on the puzzle icon again -> Userscripts -> tap on the cloud with a downwards arrow. You might need to be on kbin.socal to do that; I'm not entirely sure, to be honest.