If you are flashing official roms then no need use twrp, xiaomi own flash tool can handle. Only if u plan to flash customs then only use twrp

When u get your china rom phone, first thing is to get your mi account registered. The same username and password will be used for all things xiaomi, including its forum. Make sure the same username and password is keyed in under accounts in phone settings.

Once that is done, there is a link in forum for you to request unlock. Wait for approval sms. Once sms is received, then in phone activate developer options and then bind your mi account to the phone, under "unlocking" which is located inside developer options. Separate than the "accounts" in settings as mentioned earlier. For binding u need to use vpn to set your internet connection to china.

Once binding done u can delete off the vpn and then plug the phone to your pc, and use miunlock programme (downloaded from forum) to unlock. It will say unlock failed, then give you a prompt to try again after 360hours.

Wait until after countdown finish (u sendiri track) then re-do the miunlock thing and your bootloader will be unlocked successfully. When that is done, hopefully the official global rom is available already and u can download the rom and then use miflash tool (downloaded from mi forum again) to flash in your global rom, overwriting the china rom.

**edit**

The steps above is for phones with official china rom. Stay away from unofficial "global" rom. U will get headache trying to flash that one.