Make sure that you have 12.5.16.0.RKLMIXM installed. This is needed since the ROM won't boot on MIUI 13, because of the OSS vendor being based on MIUI 12.5. Check #firmware for more infos.

Flashing
• Boot into fastboot
• Run: fastboot flash boot <boot>.img
• Reboot into recovery via fastboot reboot recovery
• Click on Apply update › Apply from ADB
• Run: adb sideload <ROM>.zip
• Reboot back into recovery
• Flash GApps the same way you flashed ROM (optional) and reminders if your ROM includes GApps no need to flash GApps.
• Now, format data and reboot into system.

Dirty flashing
• Reboot into recovery
• Click on Apply update › Apply from ADB
• Run: ADB sideload <ROM>.zip
If you flashed GApps:
• Reboot to recovery
• Re-flash GApps with the same procedure

Notes
• Make sure that your device is ready for flashing ROMs. As in, unlocked bootloader and disabled VBMeta, along with verity check fix applied. Ask on your device's group for more info.
• To flash Magisk, you must patch the ROM's boot.img. Follow #magisk for more info.
• For Google Apps, we suggest using Bitgapps, even though you can try other packages at your own risk. Follow #gapps for more info.
• addon.d doesn't and will never work on devices with Virtual A/B. So please, don't report such issues as Lygisk or NikGapps disappearing after update, because it's known. Instead, re-flash GApps and/or Magisk normally before rebooting into system.