Seems like the plugin doesn't have a github, so you have to download and upload the script and css files using this Github hosting tutorial.
Then, following the sections in the plugin tutorial https://codyhouse.co/gem/page-scroll-effects/,
How it works - You need to add custom attributes to body
Creating the structure - You will need to re-structure the page following the tutorial (follow exactly including the class names and tag names and element hierarchy)
Adding style - If you have followed the tutorial exactly up to this point, you should be able to paste the code in this section into the Page Settings > Header Code, and wrap them between
If it's too difficult to use, search for a plugin with a simpler tutorial (like fullpage.js, which has a tutorial on Webflow forums), or simply hire a web developer.