Download the ICS stock file from
one of the links in this post.
Unzip (extract) it somewhere. Make sure on your phone USB Debugging is checked in settings>applications>development.
If you don't have Odin,
get it here. Extract it in a folder somewhere. I made a folder called Galaxy S2 to put all this stuff in.
Now use Odin
as in this tutorial except you don't need all the steps.
1 - Launch Odin but do NOT connect the phone yet. Plug your USB cable to your computer but
not your phone yet.
2 - Power off your phone. Then hold down
both volume buttons and plug in the USB cable. Press the
volume up button when you see this menu.
You will then go to the download screen.
3 - Wait for the Com port in Odin to turn yellow. You do not need to check anything that isn't already. Just be sure that Re-Partition is
NOT checked. It shouldn't be.
4 - Select
PDA in Odin and browse to the file you downloaded and select it. Now press
Start and Odin will flash the stock rom. When it is finished you will see a green
PASS in the top section. It took me about 6 or 7 minutes.
5 - Odin should reboot your phone. You should now be in the stock ICS ROM.