• Download the ROM suitable for your device from the MIUI site and extract the downloaded archive
• Download and extract the Android SDK Platform Tools
• Make sure adb and fastboot (components of platform-tools) are in your path

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

• Connect your device to your computer using a USB cable and enable USB Debugging in your device's settings. 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
• Make the script executable (I've chosen flash_all.sh)

$ cd path/to/extracted/ROM/archive
$ chmod a+x ./flash_all.sh

• Run the script

$ ./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.