First of all remember that you are the only one responsible if anything goes wrong with your phone.
You will first need to:
1. Root your device
2. Install a Custom Recovery such as ClockworkMod recovery
3. Download CyanogenMod 9 stable from here for Samsung Galaxy Note (AT&T).
Now you have to follow the next steps:
1. After you download the Samsung Galaxy Note (AT&T) ROM, move it to your phone’s SD card.
2. Reboot the device in recovery mode (Hold Volume Up, Home, & the Power button, continue to hold all three until the screen flashes, then release all buttons).
3. Use the “backup & restore” option to backup all your data.
4. If this is the first CyanogenMod installation apply the “factory reset” option in your Galaxy Note Settings Menu.
5. Now apply “wipe cache”.
6. In the “advanced” menu find “wipe dalvik cache” and apply it. Return to main menu.
7. Find the “install zip from SD card” option and click it. You will be prompted with a message to “choose zip from sd card”. Choose the downloaded zip file with the Samsung Galaxy Note (AT&T) ROM.
Wait a little before the reboot is finished. You now have CyanogenMod 9 installed on your Samsung Galaxy Note (AT&T).
You can also watch this video tutorial that shows you how to install CyanogenMod 9 ICS 4.0.4 on Galaxy Note N7000 Firmware Update.