1. Hi. Start by turning off the phone. To do that, press and hold the Power button.
2. When the phone is turned off, press and hold Volume Down.
3. Still holding the button, connect the phone to the PC via a micro USB cable.
4. You should relase held keys as soon as the desired mode appears on the screen.
5. Superb, you are now in Fastboot mode. Navigate this mode using Volume Keys; select options with Power button.