5. Check if your bootloader is locked
a. After successful installation of "ADB and Fastboot" enter the fastboot mode by holding together "Volume Down + Power" and connect your device to your PC via USB cable (best would be the original cable).
b. Go to the folder where you have put the "ADB and Fastboot" and open a command window inside that folder. To do that hold together "Shift + Right Click" on any empty white space inside the folder and then select "Open Command Window Here" or "Open PowerShell Window Here"
c. Enter the terminal command in command line: >>
fastboot devices<<
If you don't see your device it means that the drivers weren't installed properly or the device wasn't connected properly. If this happens please reinstall your drivers from Point A. 1. or try these drivers here:
http://xiaomiadvices.com/download-xiaomi-usb-drivers/
d. If you see your device then enter the terminal command in command line: >>
fastboot oem device-info<<
If your bootloader is locked, you'll get the following status (or something similar):
…………………………………………………………………….
(bootloader) Device tampered: false
(bootloader) Device unlocked: false
(bootloader) Device critical unlocked: false
(bootloader) Charger screen enabled: false
(bootloader) Display panel:
OKAY [ 0.058s]
finished. total time: 0.063s
………………………………………………………………………
If your bootloader is unlocked, you'll get following status (or something similar):
…………………………………………………………………….
(bootloader) Device tampered: false
(bootloader) Device unlocked: true
(bootloader) Device critical unlocked: true
(bootloader) Charger screen enabled: false
(bootloader) Display panel:
OKAY [ 0.053s]
finished. total time: 0.055s