I made some photos going through Step 13. I hope it's clear.
This is what Step 13 looks like in the book. The flap on the right is slightly lifted up, with the letter A at its bottom left corner.
Lift the flap up more to the right to look underneath:
I've labeled the hidden corner B. This corner needs to be turned inside-out from behind.
Now flatten to the left:
I hope it is self explanatory. Let me know if you need more help with this.
I'll also warn you that there are some more difficult steps coming up in the model. This model is not too easy, but the end result is worth it too.
EDIT: I fixed the pics posting problem