Open Bootloader

First of all, this is a longer process. An application has to be sent and approved by Xioami.
So it's a good idea to do this before you buy the phone, so that by the time you get it, you have permission to bootload it.

Submit an application:

1: You need to log in to your Mi Account (if you don't have one, you need to register first): link

2: Click on the Security tab and check if the Security check-up is 100 points (if not, fill in the missing part(s))

3: Visit the http://en.miui.com/unlock/ website, where you click on Unlock Now.

4: The opening requesting page must be entered:
-a laughs
(do not contain such characters as: number, period, hyphen, dash, or any other character: @ ! # etc.)
-you must dial the country code (according to your location, if applicable)
-your phone number (without country code)
-and at the end a short description of why you want to open the Bootloader. This is only accepted in English.
Pl: "I would like to root my phone." or "I would like to use unofficial rom"
Then you need to tick the box in front of the text "I confirm that I have..." and click on the "Apply Now" button.

It will then send a text message to the phone number you entered before, and a new page will load where you have to enter the code you received in the text message.
If it still doesn't come after the second Resend, you should try this first: link
(You can send up to 10 SMS messages per day, so you can try 10 times)
Finally, click Next at the bottom and your application has been successfully submitted.

From the day you send it, you can count 3-4 days until you get the permit!
Text messages should be coming about acceptance, but the experience is that they don't.
But you can check the status of your application like this:
link

Bootloader opening:

Regardless of the ROM version (official or unofficial webshop), try to open the bootloader.
If not, there is a description at the bottom of the page that will help you to upload something that works in 100%.

If accepted, the http://en.miui.com/unlock/ On the Unlock Now page, click on the Unlock Now button and the Download Mi Unlock button will appear on the next page.

Click on the button and MiFlashUnlock.zip will download.
You unpack it and install it.

Then the i.mi.com on the BL page to check if the phone is connected to the account you sent the BL opening request from.

If yes, you can start MiFlashUnlock and log in with the account you requested to unlock.

If you're not sure if the correct driver is installed on your phone, you can do this by clicking on the small gear in the top right-hand corner and update the program if possible

Turn off the phone, then put it in Fastboot mode (volume down + power button at the same time).
You connect it to the computer and wait until the program says "Phone connected". Then you click Unlock.

The process goes down (max 10 seconds) and you can reboot your phone with "Reboot phone".

Then you're ready! Now you can install TWRP and ROM!

Comments:

With Mi account you can only open 1 device per 30 days.
If the phone is encrypted, opening the bootloader will erase all user data.