Create an “A-record” at your DNS provider for www.yourdomain.com and yourdomain.com that points to 18.104.22.168. (You can also do the same if you’re using a subdomain, of course.)
The exact method for creating A-records is different depending on your domain registrar or DNS service.
Once you’ve created the correct DNS records, add your domain to the Blog section of Settings. Type only “yourdomain.com” or “blog.yourdomain.com” (don’t add “www.” or “http://”) in the “custom domain” box.
If you need more help: firstname.lastname@example.org.