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 and Power button for a moment.
3. Bootloader mode menu will appear on screen. When it does, let go of held buttons.
4. Choose Fastboot Mode by pressing the Volume Up button to scroll and Power button to select.
5. Superb, you are now in Fastboot mode. Navigate this mode using Volume Keys; select options with Power button.