I am personally a fan of attempting to reproduce natural conditions for most of my pets. So I "winter" my Tegu and most of my other reptiles to some degree or another.
The two things I do to replicate winter is I shorten the amount of hours per day the lights are on as well as lower the temperature of the enclosure. I suggest you monitor the weather conditions of Northern Argentina this summer (their winter) to see what kind of temperatures his native zone experiences.
Having been through 3 winters with my Tegu, and each one going very differently, I've formulated a very simple approach. I watch my Tegu, and supply him with the conditions to do what it seems like he 'wants' to do.
In the fall, if/when my Tegu starts sleeping longer hours, I begin shortening his day and lowering his temperature. If/when he sleeps through full days I ensure the temps are low enough to slow his metabolism. But if he seems to 'want' to be active when awake, I give him warm day time temps.
All in all, my best advice is simply... follow your Tegus lead...
The year when my Tegu did go into a full hibernation, I turned all heat sources off and moved his enclosure into a garage where overnight temps were around 55~60*F and daytime temps stayed 70*F or below. I left his UVB light on a minimum of 6 hours per day, even when he wasn't coming out at all. Though I turned his heat lights off entirely.
I think one common mistake people make with Tegus (or any hibernating pet) is we do not cool the enclosure enough while the animal is sleeping. Their metabolism is temperature controlled. So if we leave the enclsoure warm, their metabolism burns energy reserves unnecessarily fast and can allow the animal to starve to death while sleeping.