How to configure a custom IdentityUser for Entity-Framework

I am getting accustomed to the ASP.NET Identity framework and let me just say that I love it. No more boring hassle with user accounts: all the traditional stuff is already there. However often you’ll find yourself wanting to expand on the default IdentityUser class and add your own fields to it. This was my use case as well here and since I couldn’t find any clear instructions on how this is done exactly, I decided to dive into it especially for you! Well, maybe a little bit for me as well.