Installing WordPress on ngix in a subdirectory next to main website

I am bit stuck with the following scenario:

I am running a VPS with a LEMP-stack (Ubuntu 12.04 server, Nginx, MySQL, PHP-fpm). I changed my root directory to /var/www and succesfully managed to install WordPress in root following this tutorial, so that it is directly accessible from the main domain.

However, I would like to run my main website, which is Non-Wordpress, in the root directory (accessible via example.com), while installing my WordPress-site in subdirectory of that domain (which should be only accessible on example.com/wordpress).

I just spent 2 hours trying find a decent explanation on Google that covers this scenario. I would appreciate if someone could explain or show me a tutorial how to set this up (including correct nginx configurations).

Thanks a lot for your advice.