the link by polv-san explains it in more detail, but basically you need:
- a script manager (e.g. tampermonkey)
- the wanikani open framework (a userscript which provides functionality to other scripts)
- and a reorder script (i use srs reorder buttons)
if you do not feel comfortable installing stuff like this in your browser and messing with scripts, it might be easier to use the new built-in function to sort reviews by level