This script is essentially a stripped out version of @seanblue’s wanikani-dashboard-srs-and-leech-breakdown to only include the srs sub-stage counts, which gives a pretty basic example of using the WKOF to pull data, parse, and then manipulate the DOM with the data if that helps.
The hardest part of starting for me was figuring out how to even get started using a script manager (tampermonkey) and how to distribute scripts (greasyfork). This blog post covers that pretty well I think. Easiest thing to do is to install a script, familiarize yourself with how it works in tampermonkey, and tweaking it in some way.