Np, merry xmas
Sent from my MI 9 using Tapatalk
To flash your Cepheus from official ROM to custom ROM (xiaomi.eu), in a nutshell, this is what I did.
1. Download ADB, Mi Unlock Tool, official TWRP for Mi9/Cepheus from twrp.me, xiaomi.eu beta ROM
2. Download official Mi Flash Tool, latest Mi9 global fastboot ROM (just in case you want to flash back to official or ****ed up)
3. Backup data by syncing with Mi Cloud or Google Drive, or whatever tool or method you prefer
4. Enable developer options, then enable OEM unlocking in developer options and enable ADB debugging
5. Shutdown your phone
6. Press and hold Vol Down + Power button to enter fastboot mode
7. Unlock your bootloader (you may have to wait for the timer to allow you to unlock the bootloader). If there is a timer, reboot your phone and use it normally, then try unlocking it in a few days
Note: unlocking the bootloader will erase everything in the internal storage and your phone will be factory reset
8. After unlocking the bootloader, reboot, connect to wifi and enter your Mi account credential to activate your phone
9. Skip the setup process
10. Repeat step 4, 5 and 6
11. Run Command Prompt in your desktop PC
12. Type
fastboot devices to make sure your phone is detected
13. Type
fastboot flash recovery twrp.img (I renamed both the twrp and rom zip files to something short, such as twrp.img and rom.zip)
14. Once done, press and hold Vol Up + Power button to enter recovery. This will boot TWRP, instead of the stock recovery
15. Type
adb devices to make sure your phone is detected
16. Type
adb push rom.zip /tmp/ to sideload the rom (Note to sideload the file to tmp and not to the internal storage as we will be performing a factory reset prior to flashing the rom)
17. Go to Wipe menu, select Format Data, type
yes (This is a very important step)
18. Then do a Factory reset
19. On the main TWRP page, select Install, go to the /tmp folder, select rom.zip, and swipe to install
20. Once flashing finished successfully, wipe dalvik and reboot the phone to system
You should now be running xiaomi.eu v9.12.19 in your Cepheus.
I took the opportunity to flash my bootloader locked, official MIUI 11 v11.0.3.0 Mi 9 to xiaomi.eu v9.12.19 while writing this post.