[NS4G] Stock ROMs | How To Root/Unroot | OTA Files & Images - xda-developers
I used that tutorial. Basically idiot proof. I say basically because there are a few things it doesn't explain.
1. How to install the developer tools.
2. recovery-clockwork-3.1.0.1-crespo4g.img should be wherever you're currently looking, or in a folder in your PATH. For example, I have EVERYTHING in /Developer/SDKs/Android, and ./tools and ./platform-tools are in my PATH, so I was able to put the file in ./platform-tools and run it.
If you're wanting to flash a custom ROM, and not just a rooted stock ROM, you can either substitute the GRJ22 ROM with your ROM of choice, or flash the GRJ22 ROM and use ROM Manager afterward.
If you don't have ANY idea what I'm talking about, this one is pretty idiot-proof too:
[GUIDE] Nexus S 4G Simple Root/Unroot - xda-developers