As noted in the tutorial, you must replace the highlighted part with the actual path of your files.

    server_name example.com;
    ssl_certificate /home/sammy/example.com.chained.crt;
    ssl_certificate_key /home/sammy/example.com.key;

This snippet assumes you've uploaded the certificate into the home directory of a user named sammy Though the location is arbitrary. You can put them in any directory you choose.