How To Flash A ROM
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A ROM is basically a customized version of the Android OS. ROMs can offer performance tweaks and added features. It is always recommended to have a good battery charge before attempting to flash anything!

Instructions

  1. Read the install instructions provided by the ROM's developer.  Some ROMs require multiple files be flashed when installing a ROM
  2. Backup your Current ROM
    1. Backup your current ROM Click here for instructions
    2. Backup home screens and apps Click here for instructions
    3. Backup contacts to your Google account
  3. Download your desired ROM to your phone.
    1. I like to create a folder at the root of my SDcard labeled ROM (/sdcard/ROM). Place the downloaded ROM into this folder.
  4. If needed, download any additional files provided by the ROM's developer
    1. If you do not flash all the needed files, such as GAPPS, you will have problems.
  5. Boot into Recovery
    1. Turn off the phone
    2. Boot into fastboot.
    3. Hold down the Volume Up + Volume Down + Power until you feel the phone vibrate.
    4. Press Volume Up or Volume down until you see the arrow with “Recovery”, press the Power button to Select.
  6. Wipe Data (This will NOT erase your sdcard)
    1. Wipe data/factory reset
      1. Select “wipe data/factory reset” > Select Yes
    2. Wipe Cache
      1. Select “wipe cache partition” > Select Yes
    3. Format System
      1. Select “mounts and storage
      2. Select “format /system” > Select Yes
    4. Format Cache
      1. Select “mounts and storage”
      2. Select “format /cache” > Select Yes
    5. Format Data
      1. Select “mounts and storage”
      2. Select “format /data” > Select Yes
    6. Wipe Dalvik
      1. Select “advanced”
      2. Select “Wipe Dalvik Cache” > Select Yes
  7. Flash the ROM
    1. Select “Install zip from sdcard”
    2. Select “choose zip from sdcard >
    3. Locate & Select the ROM zip file> Select Yes
  8. Flash any additional .zip files needed
    1. Select “Install zip from sdcard”
    2. Select “choose zip from sdcard
    3. Locate & Select the zip file > Select Yes
  9. Reboot!
    1. Be very patient here. The first reboot can take longer than usual to boot. This is why it is good to start with a good battery charge. The first boot may take 10 – 15 min. You may think you are stuck at the splash screen, just wait.
    2. Follow the setup prompts, verifying that sync to your Google account is selected.
    3. The Play Store will automatically start downloading apps to your phone. Wait for this process to finish
  10. Restore Apps & Homescreens
    1. Click here for instructions
  11. Go to Settings > Accounts & sync
    1. Setup your accounts (ie. Haxsync, Facebook, Twitter, Dropbox)

Alternative Wipe Option For Step 7 [easy]

The developer of the Android Revolution HD (ARHD) ROM, Mike1986, has made a Super Wipe script for the Galaxy Nexus.  When you flash the Super Wipe script it will perform all the necessary wipes in step 7.

Mike1986 Super Wipe Script: Download Link / Mirror Link

Download the Super Wipe script to the phone before step 6 of this guide.  The instructions below will replace step 7 above.
Flash the Super Wipe Script
  1. Select “Install zip from sdcard”
  2. Select “choose zip from sdcard >
  3. Locate & Select the Super Wipe Script zip file> Select Yes
  4. Once complete, continue to step 8