I've posted some links below that have helped me understand their process in a little more depth, especially the interview. These links have been posted at some point on HN, but I'm just putting them here for anyone's convenience. I hope you find them useful:

http://old.ycombinator.com/howtoprepare.html

http://ycinterview.com/

http://ipaulgraham.herokuapp.com/

https://www.aptible.com/blog/y_combinator.html

http://yourstory.com/2014/10/guide-list-y-combinator/

The last one is especially helpful, it should answer just about any question and has other examples of great applications.