Hello, you only need :
-the twrp 3.0 for xiaomi note pro
-the room android 6.0 by mr.ivan
- gapps (you can take from this website opengapps.org (arm64 - android 6.0 - pico)
- the fix gps (i take that from xda-developers.com)
Now with this it´s easy, you install twrp - > wipe the phone and install android 6 - gapps - fix gps .When you boot your phone, you will had problems with google services ( it´s normal) , continue with the settings and when you can go to settings open apps setings and give all permisons to google services.
When you do that , you will had your phone with android 6 .