Do I need to install firmware?
currently my Android is 11.
Not necesary you seems to have the last global Xiaomi firmware, it's a bad idea sometimes to flash newer china firmware. I'm using the same versiΓ³n as you but euro zone.
You just need to unblock the bootloader on the Xiaomi Page with the Xiaomi tool. When i do it, Xiaomi sends me the unlock Code after 48hours.
Download the tool TWRP that mrumais posted on the first message. Download ADBTools and ADB DRIVERS for your computer.
TURN OFF PHONE. PRESS VOLUME DOWN+ POWER at same time to boot in fastboot mode. Appears a Xiaomi Bunny.
Connect the phone to the computer with USB cable.
RUN ON TERMINAL "fastboot flash recovery TWRP-FILENAME.IMG "
When flash ends disconect USB cable and power off the phone.
PRESS VOLUME UP + POWER. Appears TWRP app. Go to WIPE DELETE. FORMAT DATA, SYSTEM RESET, DELETE EVERYTHING EXCEPT "VENDOR, RESTORE, BOOTLOADER". THIS DELETES ALL YOUR PHONE FILES. REALLY CLEANN INSTALL.
Plug the USB cable on the computer, Search the option in TWRP to mount the phone as a Storage Device. Now apears as USB Storage on your computer. Copy the CherishOS ROM (GOOGLE VERSION BETTER) to the phone using your computer.
Back to TWRP go to install Menu and search the ROM in data o Storage, sdcard, usbdata directory. DO FACTORY RESET AND WIPE DALVIK CACHE.
REBOOT THE PHONE. AND ENJOY THIS GREAT ROM.
MAYBE I MADE MISTAKES ON THE EXPLANATION. SORRY FOR MY ENGLISH AND THE UPPERCASE.