Noob question: how do I set Giscus up?

[Expertium]

Here it says "Add the following <script> tag to your website's template". But what does that actually mean?
Here's my repo, if that helps: https://github.com/Expertium/expertium.github.io. Do I need to add the <script> to _config.yml? Do I need to add it to some other file that I need to create first?