Joe made fun of me pretty much constantly as I was taking pictures of us nailing in this little piece of wood into the wall.
Here is what the corner looked like before with the piece missing:
So what you have to do is lean over and nail upside down so that you can get into the corner and still get leverage. We also found out that getting the right length nail is important. At that hard of an angle, once you get it through the wood and a bit into the wall you want to be done nailing.