1. Reboot to Fastboot Mode
Connect your phone to your PC and reboot into the bootloader:

adb reboot bootloader

2. Enter FastbootD
Once you're in Fastboot Mode, run:

fastboot reboot fastboot

Your device will now enter FastbootD.
3. Flash Boot Image

fastboot flash boot boot.img

4. Flash Vendor Boot Image

fastboot flash vendor_boot vendor_boot.img

5. Reboot to Recovery
Use the Volume keys to select Recovery Mode, then press the Power button.
Once recovery opens, navigate to:

Advanced
→ Reboot to Recovery

Wait for the device to reboot back into recovery.
6. Factory Reset
After rebooting into recovery, select:

Factory Reset
→ Format Data / Factory Reset

Confirm the operation to wipe all user data.
7. Sideload the ROM
Return to the main menu and select:

Apply Update
→ Apply from ADB

Then, on your PC, run:

adb sideload <ROM_FILENAME>.zip

Wait for the installation to finish. Once completed, reboot your device and enjoy the new ROM.