Step1: Open browser & visit official website/url http://en.miui.com/unlock/

Step 2: Click unlock now button, you will get directed to login page. Put your MI login credential here

Step 3: Fill in all details mentioned as given in below image. Don't forget to give proper reason for unlocking (Don't copy & paste available reasons on internet as it is). Ensure you put right mobile number, as verification code will be received on it, which need to be added for submitting request

Step 4: Once you submit request you will get message something like this. After this step just relax yourself & wait for confirmation message from Xiaomi for approval of request.

Rest of work will be done by Xiaomi engineers, expect your requested approvals within 3 to 21 days depending on your activeness in forum, please be patient as this is time consuming process because this is manual verification process