How to Flash a Custom ROM on Samsung Galaxy S21 Ultra 5G: Step-by-Step Guide

Understanding Custom ROMs

• 💡 A custom ROM is unofficial firmware modified by users to add features not available on the stock Samsung firmware.
• 🚀 Examples include One UI from S24 Ultra with Galaxy AI or Pixel Experience for a different look and feel.
• ⚠️ Users must download files specific to their exact phone model (e.g., SM-G998B) to avoid issues.

Pre-Installation Steps

• 🔑 Unlock the bootloader is a mandatory first step for flashing any custom ROM.
• 🧹 Remove Google account from the device to prevent FRP (Factory Reset Protection) lock.
• 💾 Back up all important data as the process will erase the phone's storage multiple times.
• ⚙️ Enable Developer Options by tapping the build number seven times, then enable OEM unlocking within Developer Options.

Installing Custom Recovery (TWRP)

• 💻 Download Samsung drivers and the AddIn tool (version 3.14.4 recommended) from hardreset.info.
• 📲 Connect the phone to the computer and boot into Download Mode (Volume Up + Volume Down + USB cable).
• 🛠️ In Odin, disable Auto Reboot and Factory Reset Protection, then flash TWRP (custom recovery) and VBMeta disabler via the AP slot.
• 🔄 After flashing, manually reboot into recovery mode by holding Power + Volume Down, then switching to Volume Up when the screen goes black.

Flashing Custom ROMs: Two Methods

• 📂 Method 1: TWRP Installation (for ROMs not using AddIn/Odin)
  • 🔌 Mount internal storage in TWRP and copy the custom ROM zip file to the phone.
  • 📲 Alternatively, use ADB sideload by placing the ROM zip in the ADB folder, opening a terminal, and running adb sideload <filename>.zip.
  • ⚠️ This method may fail if the ROM requires a specific firmware revision (e.g., stock ROM XXSEGYA2).
• 🚀 Method 2: AddIn/Odin Installation (for ROMs with AP/BL/CP/CSC files)
  • 💻 Ensure Samsung drivers and AddIn are installed, and the phone is in Download Mode.
  • 📁 Load the respective AP, BL, CP, and CSC files into the AddIn tool.
  • ▶️ Click 'Start' to flash the ROM; the phone will reboot automatically.

Post-Installation and Finalizing

• 💾 After flashing via AddIn, reboot into TWRP, go to Wipe > Format Data, type 'yes', and confirm.
• ✅ Reboot the system from TWRP to boot into the newly installed custom ROM (e.g., Beyond ROM).
• 🌟 The first boot may take a significant amount of time; setup can be completed by connecting to Wi-Fi later.