I advise you to install first MIUI 8 via TWRP doing full wipe of all data. It's not recommended to install MIUI 8 from MIUI 7 without full wipe.
Might be big issues if you install MIUI 8 without wiping data. Tell me if you need something else!
There're many tutorials.
For example you can try with this below: http://en.miui.com/thread-190608-1-1.html
You just have to go to TWRP via fastboot and install rom which will install TWRP permanently.