Android 6.0 on HTC ONE M7

The Internet offers quite a lot of step by step instructions on how to flash an Android device with a different ROM (Firmware). The most of them tell you how to do it and are really well for a defined device and a defined software. I'm a developer and I like to have some more options. I don't want to write a general flashing manual. The aim is to share some useful information about flashing.