During the unlock process I got stuck within the 'Add account and device' in Developer Options, with the error code 86006 being displayed. The solution to this issue is to change from the Stable MIUI 8 to the latest Developer ROM - once this is flashed, the device will register with no error messages.
An overview of the process is therefore:
1. Flash the latest Development ROM on the phone, via the Updater app
2. Register for a Mi account, if you do not have one, and log into the phone with this account.
3. Request the Unlock privilege for your Mi account
4. Enroll the device for an Unlock in Developer Options
5. Wait 72 hours
6. Unlock the bootloader with the Windows Mi Flash Unlock tool.
The Unlock tool will require that the phone is logged into the same Xiaomi account in order for the unlock process to be successful, so ensure you have set yourself up with a Mi account and signed into the phone using this account.
I tend to ensure I have posted a few items on the MIUI forum before I make a request as, during the request process, it states that they will take the activity of your account into consideration when they are making the decision to grant you the unlock privilege.
If the request is denied, you have to wait 30 days to re-apply, so better to be safe than sorry!
Request the Unlock privilege
The first thing you will need to do is to make sure you have applied for Unlock rights for your Mi account. This can be done here:
Try to make the given reason for unlocking something specific, rather than generic, as your request is more likely to be granted this way.
The time between application and hearing back from the MIUI team seems to vary. The last time I applied the request was granted after just a few minutes, but previous applications have taken days!
Setting up the phone
Before you can successfully unlock the phone using the unlocking tool you will need to enrol the phone into the scheme, which is performed in Developer Options.
1. Go to Settings / About Phone
2. Press MIUI version 7 times to enable Developer options
3. Go to Settings / Additional Settings / Developer options
4. Click on Mi Unlock status
5. Click on Add account and device
Note that if you get an error code of 86006 then download and flash the latest Developer ROM through the built-in Updater app:
Xiaomi ROM Download
Three Day Wait
After enrolling the phone, Xiaomi require that 72 hours (3 days) must elapse before the bootloader can be unlocked with Xiaomi's MiFlash Unlock Tool.
If you attempt to unlock during this period, the process will get to 99% before displaying an error message requesting you wait 72 hours.
Unlocking the bootloader
Once the phone is enrolled and 72 hours have passed, you can use the Unlock tool.
This can be downloaded from the MIUI Unlock site once your unlock request has been granted:
On a Windows PC, extract the zip file to a temporary folder and run the miflash_unlock.exe tool.
Log on using the same Mi account that you have signed into the device with and follow the on screen prompts, which involve booting the phone into Fastboot with Power and Volume Down.