Great guide. Worked great to flash in Linux. I have a few improvement proposals. I had some problems understanding how flashtool works. It became clear once I booted up the phone into Android and then run Flashtool and plugged the cable. The phone was recognized and there were instructions how to power it off and press volume-down when plugging in the cable etc. Please add that you need to press volume-down to enter flash mode while plugging the USB cable. This is also explained when using Flashtool.
Hold volume-down to enter flash mode here (not fastboot mode). It is true that led is blue in flash mode.