Here are the modified steps you need to get this working in Webflow:
Copy css to your document or link to the css file in header
Instead of this, paste this in Site Settings > Custom Code > Header Code:
<link rel="stylesheet" href="https://cdn.rawgit.com/kni-labs/rrssb/master/css/rrssb.css" />
Copy .rrssb-buttons unordered list to desired location(s):
You need to insert this in an embed code component where you want the buttons to be.
Instead of this, paste this in Site Settings > Custom Code > Footer Code:
Publish and view with published link.
You cannot view this in the designer.
Also, feel free to contact me for further code help and/or customization of third-party plugins