once they look nice and fat and once they stop eating or moving around as much, they're ready for hibernation. first take them off heat, next let them rest in room temp for a couple days or so, and then put them in the fridge.
camponotus can handle near freezing temps and the standard fridge works great for them! Put in the back, place on a towel to minimize disturbance, keep the nest dark- use aluminum foil, and don't bother them for 3-4 months. shoot for 4 months.
your garage maaaaay be ok but why chance it? why potentially kill a year or more of work raising your colony? we keep these ants in controlled environments the rest of the year so why expose them to the elements for 3-4 months. my camps are from high up in the mountains, where there is plenty of ice and snow in the winter. there is no way my garage in sunny los angeles could mimic that, but a fridge can!