First off, I'm not responsible for any damage to Your devices, I'm sharing my experience and it should not be taken as a replacement for manual and common sense. You're following the guide at Your own risk!!
The purpose of this document is to help understand me the process of flashing. The issue with a lot of guides is that they describe only the necessary steps, and provide pre-patched archives and images, pre-downloaded binaries, which is not a very assuring or secure way of approaching things. I will try to describe where to get original binaries, how to patch the original image.
Don't treat the document as an oracle, go check and verify things. Cautious is the way!