Just to add my first and recent experience here...
First, however, I'd like to give my thanks to all that contributed the great input/tips!
I pulled out my airbox. This allowed me to see the CTS directly.
Not sure if this tip worked/helped, but I unscrewed the overflow tank's cap, squeezed and held the upper radiator hose while screwing the cap back down. I thought it was a great tip, even though I'm not sure of how effective it was: I didn't really lose that much coolant, so perhaps it did the trick? In case anyone reading this missed this tip earlier in this thread, the idea is to create a vacuum in order to reduce coolant loss.
I was able to pop the clip out from the front of the engine using a long small-ish flat blade screw driver. I made sure to hold the old CTS in place. Hold down a bit on the top of the clip with your "free hand" and you can keep it from flying off: I had a new one so I wasn't worried about losing it- BUT, putting the clip back on presents a fumbling possibility in which you could lose one while trying to stick it back on!
Wire harness was left connected as I wasn't successful in dry run attempts to disconnect it. MUCH easier to deal with with it pulled away with the old CTS.
I slowly wiggled the old CTS out, stopping after only just a little movement as I watched to see if the O-ring was coming out or not. Having the airbox out allowed me to see th O-ring make it's way out as it passed the slot that the clip goes in to. I was fortunate that the O-ring did come out- it was kind of half-on, kind of skewed. Not sure if wiggling the CTS a bit helped dislodge it from its seat in the housing or whether it resulted in starting to slip it off the CTS.
New CTS (with lubed O-ring) went in, round side up (think "RoundUp" - ugh!). Getting the/a clip back in seemed, to me, more awkward than anything else. It goes in from the top (if you were paying attention to how the original one came out) and I found that I kind of had to lean it in as I neared the groove in the housing and then stand it up and pop it down. It did pop down rather easily, almost too easily, which made me wonder whether I really had done it correctly- I had. Was a bit nervous here because I was needing to use the car right away (long drive) and I would have liked to feel a bit more sure about it: that no coolant was coming out was enough proof, and that it didn't pop out once I ran the engine for a while (pressure built up).
I wrote up the exact steps that I was going to take and made sure I stuck to the plan so that I didn't panic in the middle of things, if I encountered a lot of coolant pouring out!
One possible new tip- I stuffed a large old towel underneath the CTS. No idea how people were sticking zip-lock bags down there to catch coolant! I ended up getting just a small amount of coolant loss: I could have gone without topping off, but I'd had some extra pre-mixed and topped off anyway: maybe a cup?