I followed these instructions (didn’t even know what a userscript was when I started…) and had no problem with them once I realised I couldn’t install them on my Samsung tablet!
Edit: Sorry, just in case you haven’t got the actual script lined up, this is the one I’m using: WaniKani Lesson Ordering II (obsolete, does not work)
There is also Reorder Ultimate which lets you reorder both lessons and reviews, but it is more complicated to implement, and I don’t see the need to reorder reviews as it tends to remove the interleaving that is there to help the items lock into your long-term memory better…