1. Go here and download the latest ZIP release of the ghost default theme Casper.
  2. Setup your Disqus account here.
  3. After signing up go here.
  4. Select your Disqus plan.
  5. On the installation page press 'Ghost Platform', in the window that opens, copy the 'universal code'. Disqus will automatically configure all necessary settings for your site.
  6. Extract the Casper theme in your downloaded folder.
  7. In the extracted folder, open post.hbs.
  8. Paste the copied code from Disqus between {{#post}} and {{/post}} and place that code anywere on the page were you want your comments to appear. I chose to have my comments below subscriber window.
  9. Select the extracted casper theme folder and compress the entire folder, rename the folder so that you recognize that it contains Disqus comments. Something like 'Casper 1.3.6 disqus' will do just fine.
  10. Upload the updated Casper theme to your website yourwebsite.com/ghost/settings/general/ and activate.
  11. Go to a recent article on your website and check if everything is working!

Hope this helps.

Additional resources:
support.ghost.org/disqus