Posted: July 29, 2012
Note: this was initially written specifically for SpazeDog roms, but as they have not been updated in a while I have provided links to the Evervolv roms as well. If you would like to try a different ICS or JB rom this guide should work, just substitute with the appropriate rom and depending on the one you choose, you may also need to install Google apps (if not included).
Disclaimer: This guide involves making changes to your phone that potentially could brick your phone leaving it unusable. I am not liable for any changes you make or problems you may have with your phone. You should at least be familiar with entering commands using command prompt. This procedure will erase ALL the data on your phone. Make sure you have a backup of everything. A nandroid backup is recommended. Proceed at your own risk.
Before we get started make sure you have the following: a rooted Nexus One, a USB cable to attach your phone to a computer, and the Android SDK. If you are just now installing the Android SDK, open the SDK manager and make sure to install the "Tools" and in "Extras" the "Google USB Driver" as shown in the screenshot. Alternatively, you just download ADB and Fastboot (just google "ADB Fastboot Download"). If you have a Mac see this thread to get started with ADB and Fastboot.