Go for wordpress plugin if you confuse from fb dev docs.
WPBeginner already write amazing tutorial on that.
You’re confuse because you don’t know some specific things on their doc, otherwise they have already given all steps very clearly, even all fb comment plugin developer get the ideas from origial source.
But here is some quick steps, if you want to add fb comments from scratch.
First create a new facebook app.
Go to comments plugins configurator, scroll down till the example comments over then click on get code button.
Then in first step select the app, which you;ve created earlier.
<body> tag on your wp templete and copy paste the step 2 codes.
Add the div codes at the end of blog post . You can add div codes directly from your templete, or even you can use HTML widget, then drag the HTML widget to below blog post. It is same like you display adsense ads at the end of blog post.
Personally I would say go for plugin if you don’t have experience in web development.