Paste the code to you Publii plugin’s HTML field.
Next, go to your Publii plugin and find the HTML field. It is most likely in the form of an HTML/Code widget or module which you can find on Publii’s dashboard. Add it to your page, and paste the code you’ve copied before to the HTML field. Save and update, and that’s it, you should have Common Ninja’s RSS Feed website on your Publii plugin.