Enter the command adb reboot bootloader

Wait a couple seconds until your device rebooted into the bootloader, then enter the command fastboot flashing unlock

Some devices might use the command fastboot oem unlock instead.

If you receive an error message along the lines of FAILED (remote: 'Failed to unlock, decrypt failed!'), your phone has a locked bootloader and needs an unlock code from the manufacturer.

If you're stuck at < waiting for any device > and have already confirmed that your connection is stable, you may need to update your phone's USB driver.

You should now see a prompt similar to the one shown in the picture. Use the volume rocker to choose the Unlock the bootloader-option and confirm by pressing the power button.