This is awesome Adrian - I wish this existed six months ago. I've been asked this many times lately, and I think it's important enough to add it here as well:

The best way to prepare for the YC application process is to actually work on your company.

You learn so much about your customers and product space by actually building something and talking to customers about it. By doing that, thinking critically about the results, and iterating, you'll find the YC application and interview questions get easier to answer - because you'll know more about what you're doing and why than anyone else.