You'll need to setup a Disqus account as well as add the domain you want to setup Disqus on. You can setup an account via this link: https://disqus.com/profile/signup/.
- Log into Disqus once your account is created and select the Installation menu.
- Choose Ghost and copy the Universal Code snippet provided.
- Log into your Ghost instance and download your current theme.
- Unzip and open the post.hbs file.
- Place the snippet where you'd like Disqus to load.
- Zip up your theme and rename accordingly. (Ghost won't overwrite a theme with the same name.)
- Once uploaded just activate and Disqus will load independently on each post page.
- Placement is key, you can move the snippet around until you find a good fit on your blog.
Keep in mind the free tier of Disqus does contain ads but you can upgrade if you wish.