Download twrp libra 7.0
https://www.androidfilehost.com/?fid=457095661767135741
link for downloading the Fastboot adb:
https://drive.google.com/file/d/0B0MKgCbUM0itVVFWRC02Q0VBQnc/view
Steps:
.:
Flash Nougat rom and enter at en.miui.com/unlock to apply for Unlocking.
First you have to receive the sms with unlock code.
Then, in your computer, enter at en.miui.com/unlock and download Mi Unlock Tool.
Conect the phone to computer via fastboot mode and run the unlock tool.
For installing twrp, you have to download in your pc the twrp and the fastboot adb.
Conect the phone via fastboot, open CMD.exe, type "cd\", "cd adb" and check if it’s unlocked with this commands: “fastboot oem device-info”
If it’s unlocked, then you can flash twrp writing in command line: "fastboot flash recovery twrp.img"
Then you write "fastboot boot twrp.img" for reboot the phone.
If everything is ok, the phone will reboot in twrp.