1. Plug out your Phone (detach USB cable from phone, not computer)

2. Turn your phone off and to be certain, wait a couple seconds after the screen turned black (LED light on lower-left side has to be off!).

3. Prepare flashing process by entering the command on your computer. Wait till "Suitable USB device not found, waiting..." is being displayed.

4. Connect your phone with USB cable and wait for it to turn on automatically (it should display a white USB symbol on top right).
--> if that doesn't work, start again over with the first step.

5. If 1-4 done right, flashing process starts by it-self...

Good luck