Adding comments to Hexo is really simple.

First, create a site on Disqus: https://disqus.com/admin/create/.

In _config.yaml, add the following anywhere in the file:

disqus_shortname: SITENAME

where SITENAME is the name you gave your site on Disqus. Then:

$ hexo generate

Each post now has its own comment section if the Hexo theme supports it.