After installed miflash you need to log in with the mi account bind to your device.
Next step is to fix the checkpoint error, you then go to settings on the up left and leave the item "checkpoint" blank.
Then when you download the fastboot rom you have a .tgz archive, you need to extract the content with a zip extractor like 7zip, you then will have a .gz arvhive, you need to extract the content again. You will have a folder.
Connect you device into fastboot mode and refresh to see if it is recognized
If so now browse and select that folder
On the bottom make sure to select "clean all" on the right of that you will see "flash_all.bat" is selected, now simply click on flash on the top right and wait for the process to complete device will reboot automatically, first start will take a little more time.
In the event of antiroll back error let me now I have a workaround for that