Here is my way to install N14:

1. Unpack N12 archive!
2. Install latest kernel!
3. Boot N12 and skip the wizard!
4. Delete 'data' and 'system' folder! (as e-yes suggested)
5. Unpack N14 archive!
6. Boot N14! It will shows "Upgrading Android".
7. Enjoy!


I can use it only in the first time I boot N14. After that, I'm stucked at flashing "android" word and never reached the homescreen. Now I'm using my N12 with 2.3.7 update. This is the most stable version I think. I suggest everybody to use this version!
I suggest you to boot N12 first before you install any updates! I feel NITDroid works more stable!