Mi mix 2s. Installing without format data
0. Backup settings and applications to internal storage. Sync everything.
1. Copy ROM.zip to internal storage
2. Install latest TWRP 3.2.3-0 V8 (you can find it on XDA)
3. Boot to recovery
4. Wipe all: cache, dalvik/ART, data, system
5. Install ROM
6...