Please make a full backup first!!!

Disclaimer: I am not responsible for any mistakes that are made or for any lost data. Also if anything should break. 

Connect the smartphone to the PC.

And set the connection on the smartphone to "File transfer/Android Auto".

Open CMD as administrator

Type "adb devices" there.

adb devices

If the ADB & Fastboot drivers are installed correctly, then it will try to connect to the smartphone. This will most likely fail the first time. But this is not bad. 

A popup will appear on the smartphone. With this you confirm that you allow the connection to the pc. (Very important: also activate "Always alow from this computer").

Then just press alow and run "adb devices" again.

and everything works.  

If everything worked out, then the second time it will look like this.

Next enter "adb reboot bootloader".

adb reboot bootloader

Then enter "fastboot devices" to test.

fastboot devices

If everything looks like in the previous screenshot, then enter "fastboot flashing unlock" next.

fastboot flashing unlock

After that, the device wants you to confirm your input. 

Now you are done and can either continue with the next step or restart the device using the volume keys and power button. 

But come back here for the next step.