Download the ROM suitable for your device from the MIUI site

Extract the downloaded archive

Install the Android SDK

Make sure adb and fastboot are in your path

$ export PATH=path/to/android/sdk/platform-tools:$PATH

Check if your device is detected

$ adb devices

If device is listed, reboot to bootloader

$ adb reboot bootloader

Check if your device is detected by fastboot

$ fastboot devices

You may need to use sudo before fastboot if you get a permission denied error

There are two scripts for flashing the ROM:

1. flash_all.sh - Flash ROM and erase user data
2. flash_all_except_data_storage.sh - Flash ROM without erasing user data

Whichever script you decide to use, make sure that the interpreter is mentioned at the top of the script like

#!/bin/sh

If the above line is missing, add it to the top of the script

Run the script (I've chosen flash_all.sh)

$ cd path/to/extracted/ROM/archive

$ ./flash_all.sh

You may need to run sudo ./flash_all.sh if you had to use sudo with fastboot

Sit back and relax. The script needs no user interaction. After flashing the ROM, the scipt will reboot your device.

Reboot will take quit a bit of time. Don't panic.