I recently unlocked the bootloader on my Nexus S, and I am looking at ways to finish rooting it. Every reference I find seems to be written assuming non-Google hardware, but the Nexus S is supposed to be open.
Your best source probably is Nexus S: All Things Root Guide -- which is a compilation/collection of several guides. Here you can probably pick your minimalist solution as well as go for the "big thing". In case you need some addional sources (easily to find with a simple Google search, by the way, as that is what I just did):
Don't get me wrong: I'm not saying you didn't search (your question clearly indicates you did). I put this just for reference by other interested readers here, who might need more details.
I've just done this for my Nexus S running 4.1.2 so thought I'd let you know how I went about it.
I firstly downloaded the Nexus Root Toolkit on my computer (note that you'll need a Windows machine for this to work). From that point you can pretty much follow the instructions in the application itself and on the site, here's the instructions from the site:
Rather than going through each step in detail (the documentation and help in the application is pretty extensive already) I thought it'd be more helpful to mention the issues I came across.
This is the last screen you'll see:
The SuperUser bit is fine, the Toolkit will have installed it onto your phone for you, I just updated it using the Play store option that is presented in the application.
The Toolkit didn't seem to have installed BusyBox onto the phone but if you do want it then just go to the play store, download BusyBox and then install it.