1. download twrp img && SuperSU first
  2. make sure open the USB Debugging switch and OEM Lock in Developer options

adb reboot-bootloader
fastboot flashing unlock
fastboot flash recovery your twrp.img
(use Volume Down&Up Key to choose the recovery mod and use Power Key to confirm)
adb push SuperSU.zip /sdcard/
(install the SuperSU.zip in recovery mode using the twrp tool)