Where do I enter these codes?
Click on the little "gear" icon at the top left corner of the main screen and click "Enter Secret Code"

What do I get for them?
At the moment it is just new avatars for online play, but there may be other rewards in the future or that I do not know about.

Where did you find these codes?
Totalbiscuit made a video where he showed off these particular codes, and that video can be watched here:
https://www.youtube.com/watch?v=4ZLXrW3Hnwg

How do I know if it worked?
If the game recognized the code you will receive a pop up notification immediately, if it failed you will just go back to the settings menu.

Code:

Code:

If you know of any other codes leave it in a comment and I'll add it to the guide.
Also remember to rate and favorite this guide if you found it helpful :)